The word "son" in the New Testament was not used to support the Trinity

At the heart of the belief in the trinity lies the erroneous belief that Prophet Jesus (pbuh) is the son of God. (Surely God is beyond that!) However, when one examines how and why son was used when Christianity was born and in earlier periods, a very different picture emerges.

Son is used to refer to Prophet Jesus (pbuh) in four very different ways in the New Testament: the son of Mary, the son of David, the son of man, and the son of God.

The term son of Mary refers to the fact that Prophet Jesus (pbuh) was physically born through by Mary, and the son of David denotes his lineage.

The term the son of man is of enormous importance, both from the point of view of how he described himself and how those around him regarded him. In fact, this term appears more frequently in the New Testament than the Messiah and the son of God. The term the son of man is unique to Jewish theology and is used far more in the Old Testament, particularly in the Book of Psalms. It refers directly to human beings and is a very familiar expression. For instance, it is used 90 times in reference to Prophet Ezekiel (pbuh), who is certainly depicted as a mortal human being.

The Aramaic equivalent of this term, bar nash(a), was used for everyone also at the time of Prophet Jesus (pbuh). However, the New Testament term the son of man is not used for everyone, as in Judaic scriptures, but is employed many times to refer to Prophet Jesus (pbuh).48The term the son of man is used 69 times in Matthew, Mark, and Luke, and 13 times in John. In only one place it is used for humanity in general (Hebrews, 2:6-8). The description is used several times both regarding and by Prophet Jesus (pbuh), in the sense of I. Some of these passages are as follows:

And they were all amazed at the greatness of God. While everyone was marveling at all that Jesus did, he said to his disciples: "Listen carefully to what I am about to tell you: The Son of Man is going to be betrayed into the hands of men." (Luke, 9:43-44)

For as Jonah was a sign to the Ninevites, so also will the Son of Man be to this generation. (Luke, 11:30)

We are going up to Jerusalem, and everything that is written by the prophets about the Son of Man will be fulfilled. He will be handed over to the  Gentiles. (Luke, 18:31-32)

While he [Jesus] was still speaking a crowd came up, and the man who was called Judas, one of the Twelve, was leading them. He approached Jesus to kiss him, but Jesus asked him: "Judas, are you betraying the Son of Man with a kiss?" (Luke, 22:47-48)

As stated above, the term the son of man was used by the first Christians in the sense of human being, for most of the first Christians were Jewish and had always used this term in that sense. Its use in the Old Testament supports that view and reveals that Prophet Jesus (pbuh) was a human being created by God and in need of His mercy.
The New Testament's expression the son of God constitutes one of the so-called foundations of trinitarianism. This interpretation has led to centuries of debate in the Christian world, however. In fact, every researcher familiar with Jewish culture and language at that time has stated that the expression is metaphorical. The widespread view is this: The son of God was a metaphorical term already in wide use in Jewish society and frequently used to refer to important individuals. 
In 1977, seven biblical experts including Anglican theologians published The Myth of God Incarnate, which provoked considerable interest. In the foreword, editor John Hick, wrote:

The authors of this book are agreed on the need for a major religious development to take place in this final period of the 20th century. This need stems primarily from an increase in information concerning the origins of Christianity. Moreover, it depends on the acceptance of Prophet Jesus as a human being sent by God with a special duty and for a holy purpose. And it is based on accepting that the belief that Prophet Jesus was the incarnation of God and that he is the second element in the Trinity is a poetic and mythological expression of what Prophet Jesus means to us.49

Throughout the book, Hick concentrates on the fact that the term the son of God is a belief put forward after Prophet Jesus' (pbuh) death, one that he never communicated himself.50

Jesus did not make the claim to deity that later Christian thought was to make for him… [I]t is extremely unlikely that the historical Jesus thought of himself in any such way. Indeed he would probably have rejected the idea as blasphemous; one of the sayings attributed to him is, 'Why do you call me good? No one is good but God alone' (Mark, 10.18). Of course no statements about what Jesus did or did not say or think can be made with certainty. But such evidence as there is has led the historians of the period to conclude, with an impressive degree of unanimity, that Jesus did not claim to be God incarnate.51

Among the Jews of that time, the son of God  had the metaphorical meaning of belonging to God. Someone referred to in this way was considered close to God, one who served Him with all his heart, and who lived the kind of life that would be pleasing to Him. It never meant that the person in question possessed attributes resembling or equivalent to His, or implied any divine status. (Surely God is beyond that!) Indeed, there was no room for such a belief in Judaism.

Therefore, the first Christians may have used this term to express their respect for Prophet Jesus (pbuh) and their belief that he was His servant and close to God. In fact, many Christian theologians say that the term the son of God was given to him by his followers to honor and praise him. Moreover, they stress that it is a metaphor.

The title son was a particular form of expression in Hebrew and was used, together with other concepts, to indicate that he had a particularly close relationship to something. According to The Catholic Encyclopedia:

The word "son" was employed among the Semites to signify not only filiation, but other close connexion or inmate relationship. Thus, "a son of strength" was a hero, a warrior, "son of wickedness" a wicked man, "sons of pride" wild beasts, "son of possession" a possessor, "son of pledging" a hostage, "son of lightning" a swift bird, "son of dead" one doomed to death, "son of a bow" an arrow, "son of Belial" a wicked man, "son of prophets" disciples of prophets etc. The title "son of God" was applied in the Old Testament to persons having any special relationship with God. Angels, just and pious men, were called "sons of God"..52

Dr. Mahmut Aydn of the 19 Mays University Theology Faculty in Turkey reports John Hicks' views on this subject in these terms:

This expression was already very widespread at the time of Prophet Jesus (pbuh) and was frequently used for important individuals. Therefore, his disciples adopted the son of God and used it to describe their leader: Prophet Jesus (pbuh). Accordingly, Prophet Jesus (pbuh) was described as the son of God, just like the kings from the line of David and persons of religious and moral importance living in Jewish society. According to Hick, in Jewish thought those kings and important people who were awarded this title were never literally regarded as being God's son. Rather, they were honored by being regarded as such, and were revered and praised, solely on account of their personal characteristics. In other words, the title in question was never used in a literal sense in Jewish thought. The term son of God was used for people throughout Jewish history. The Messiah, for example, was regarded as an earthly king who would have to be descended from the line of Prophet David, regarded as the son of God… The followers of Prophet Jesus (pbuh) were able to adapt this term to him very easily. The way that Prophet Jesus (pbuh) was seen as an entity with divine attributes is first of all the result of the culture in which he lived. Accordingly, at the time of Prophet Jesus (pbuh), certain individuals with important characteristics were known as the son of God in the symbolic sense."53

After considering Hick's views in some detail, Aydn says this about the use of the term the son of God:

Neither Prophet Jesus (pbuh) himself ever suggested that he was divine, nor did the disciples ever ascribe to him any form of divinity or god-likeness. On the contrary, the idea of his divinity was put forward inside the first Christian community and gradually developed, assuming the nature of doctrine and dogma. That is because expressions regarding the divine sonship gradually left the Jewish context and entered that of Hellenistic Roman culture.54

P. M. Casey, a New Testament scholar and author of several books on the origins of early Christianity, says: "… Jesus could have been called a son of God by anyone who thought that he was a particularly righteous person."55He also draws attention to how, within the Jewish tradition, important individuals and events were referred to by mythological and metaphorical expressions, either individually or socially.56

That is how We make the Signs clear so that, hopefully, they will return. (Surat al-A'raf: 174)

According to the concept of the Messiah in Jewish belief, the Messiah, a king, would be descended from the line of Prophet David (pbuh). The kings descended from that line were regarded as "the sons of God in the sense of being appointed to the rank of kingship…"57Those who believed in Prophet Jesus (pbuh) as the Messiah may have seen nothing improper in referring to him by that term as an extension of this belief.

In The Historical Figure of Jesus, Sanders also notes that the Jews never understood the son of God in the literal sense. According to him, they used the term the sons of God in a metaphorical sense for both men and women and as a symbolic statement of one's devotion to God.58Sanders interprets  the son of Godby the first Christians in the following way:

The early Christians, used "Son of God" of Jesus, but they did not think that he was a hybrid, half God, and half human. They regarded "Son of God" as a highdesignation … The first followers of Jesus, however, when they started calling him "Son of God," would have meant something much vaguer: a person standing in a special relationship to God, who chose him to accomplish a task of great importance. When Gentile converts started entering the new movement, they may have understood the title in light of the stories about Alexander the Great, or of their own mythology.59

As Sanders states, when Prophet Jesus' (pbuh) words and deeds passed from the Jews to the Gentiles, at that time the pagan world, this metaphorical expression began to acquire a new meaning and to be used to refer to his alleged divine status. In this way, by being regarded as the Messiah by the first Christians and as having a fully human identity, he gradually became divine. (Surely God is beyond that!) William C. Varner examines how the first Christians perceived this term:

It is also true that the New Testament calls each one who believes in Jesus a "son of God." (John, 1:12) How then does Jesus being the son of God differ from my being a son of God? Does the title son of God clearly express Jesus' deity, or do Christians read more into it than the Bible intends? The only way to answer these questions is to comprehend how the first hearers and readers of the Christian message understood the title when they encountered it. Who were those first hearers/readers? On the most basic level, some were Gentiles and others were Jews. Both already had the expression son of God in their first-century linguistic and cultural backgrounds.60

Proponents of the trinity do not accept the interpretation that the concept of the son of God in the New Testament is an expression of honor and respect, even though many New Testament statements clearly use it in that very sense. For example, the sons of God is used for all who have faith in God and follow Prophet Jesus' (pbuh) path:

But I tell you: Love your enemies and pray for those who persecute you, that you may be sons of your Lord in heaven. He causes His Sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. If you love those who love you, what reward will you get? Are not even the tax collectors [of the Romans] doing that? And if you greet only your brothers, what are you doing more than others? Do not even pagans do that? Be perfect, therefore, as your heavenly Lord is perfect. (Matthew, 5:44-48)

… those who are led by the Spirit of God are sons of God. (Romans, 8:14)

I will be God to you, and you will be my sons and daughters, says the Lord Almighty. (2 Corinthians, 6:18)

Your reward will be great, and you will be sons of the Most High, because he is kind to the ungrateful and wicked. (Luke, 6:35)

As is apparent from the examples above, the term "the sons of God" is used in many passages in the New Testament. Clearly, the meaning is the same as that in which it is used in Jewish culture in general: people who have taken God as their friend, who endeavor to draw close to Him, and who live by His laws. Indeed, Luke's Gospel mentions Prophet Adam (pbuh) as the son of God (Luke 3:38). The term the son of God is also used many times in the Old Testament. For example, son is used for the Israelite people in the Old Testament:

When Israel was a child I loved him, and out of Egypt I called my son. (Hosea, 11:1)

In another extract from the Old Testament the same expression is used to describe angels:

One day the sons of God [the angels] came to present themselves before the LORD, and Satan also came with them. (Job, 1:6)

At the beginning of the Book of Exodus, Chapter 6, the term the sons of God is employed to describe humans multiplying on the face of the Earth. Other passages along these lines read: (Surely God is beyond all the expressions that follow!)

Then say to Pharaoh: "This is what the LORD says:' 'Israel is my firstborn son.'" (Exodus, 4:22)

They will come with weeping; they will pray as I bring them back. I will lead them beside streams of water on a level path where they will not stumble, because I am Israel's Lord, and Ephraim is my firstborn son. (Jeremiah, 31:9)

I will proclaim the decree of the LORD: He said to me [David], "You are My Son; today I have become your Lord." (Psalm, 2:7)

The term son of God is also used to refer to Prophet Solomon (pbuh) in the Old Testament. The relevant passage reads:

When your days are over and you go to be with your fathers, I will raise up your offspring to succeed you, one of your own sons [Solomon], and I will establish his kingdom. He is the one who will build a house for Me, and I will establish his throne forever. I will be his Lord, and he will be My son. I will never take My love away from him, as I took it away from your predecessor. I will set him over My house and My kingdom forever; his throne will be established forever. (1 Chronicles, 17:11-14)

This holy individual referred to by the title "son of God" is the Prophet Solomon (pbuh), one of the sons of Prophet David (pbuh).

[David said to Solomon, "God said to me:] But you will have a son who will be a man of peace and rest, and I will give him rest from all his enemies on every side. His name will be Solomon, [1] and I will grant Israel peace and quiet during his reign. He is the one who will build a house for My Name. He will be My son, and I will be his Lord. And I will establish the throne of his kingdom over Israel forever. Now, my son, the LORD be with you, and may you have success and build the house of the LORD your God, as He said you would." (1 Chronicles, 22:9-11)

The same chapter continues:

Of all my sons—and the LORD has given me many—he has chosen my son Solomon to sit on the throne of the kingdom of the LORD over Israel. He said to me: "Solomon your son is the one who will build my house and my courts, for I have chosen him to be My son, and I will be his Lord. I will establish his kingdom forever if he is unswerving in carrying out My commands and laws, as is being done at this time." (1 Chronicles, 28:5-7)

All of the above statements make it clear that in the Jewish tradition, the son of God was used to refer to people who took God as their friend, who were sincere and devout. This is why it was used to refer to Prophet Jesus (pbuh). Just as in the cases of Prophet Adam (pbuh) and Prophet Solomon (pbuh), it is a metaphorical term that stems from Jewish traditions. It was chosen by the first Christians, who were Jewish and who knew the Torah and lived by the Mosaic Law until becoming followers of Prophet Jesus (pbuh), to express Prophet Jesus' (pbuh) respect, devotion, and closeness to God.

Another piece of evidence showing that the term the son of God provides no basis for belief in the trinity concerns the use of the name of God in the New Testament. In his paper "Who is Jesus? Do the creeds tell us the truth about him?" Anthony Buzzard writes:

Thousands upon thousands of times in the Bible (someone has calculated over 11,000 times), God is described by personal pronouns in the singular (I, me, you, He, Him). These pronouns in all languages describe single persons, not three persons. There are thus thousands of verses which tell us that the "only true God" is One Person, not three. There is no place in the New Testament where the word "God" can be proved to mean "God-in-Three-Persons." The word God, therefore, in the Bible never means the Trinitarian God. This would immediately suggest that the Trinitarian God is foreign to the Bi.61

As we have emphasized throughout this chapter, the term son was widely employed in Jewish culture and bore no divine significance. Therefore, those who use it in the belief that Prophet Jesus (pbuh) was literally the son of God, who claim that he has powers equal to God's (Surely God is beyond that!), are making a serious mistake in terms of Jewish belief. The Qur'an warns people several times not to use this term to ascribe divine status to Prophet Jesus (pbuh). This is a grave sin in the sight of God. For example:

Those who say: "God is the Messiah, son of Mary," are unbelievers. Say: "Who possesses any power at all over God if He desires to destroy the Messiah, son of Mary, and his mother, and everyone else on Earth?" The kingdom of the heavens and Earth, and everything between them belongs to God. He creates whatever He wills. God has power over all things. (Surat al-Ma'ida: 17)