Second Boy, Trinity is totally out of logic for a non-Christian (well to me even for Christians too, but they are accepting as it is, they don't question it naturally). I don't think that people can explain it to you. it is not a straight forward belief, but very tortuous. It was probably put as a distinction from Judaism, but this not only issue untill 4th AD century many things changed. The original teachings of Jesus were already forgotten by "Christians" who were a new group formed despite Jesus didn't intend to bring a new religion:
Matthew 5: 17"Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. 18I tell you the truth, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished. 19Anyone who breaks one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven, but whoever practices and teaches these commands will be called great in the kingdom of heaven. 20For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven.
It is quite clear he didn't want to put the people into another belief, but he wanted to correct them by being a very god sample who fulfils the law of God (OT). But it changed after Paul (well, even we are not sure scholarly that all those letters were written by him). Though they are trying to find some good reasons for that, to me as a seek of being different than Jews, they gave up using some of commandments of OT, i.e. circumcision (Gen. 17.10-27; Gen. 21.4; Gen. 34.14 in many places etc...), pork prohibition (Lev. 11.7; Deut. 14.8) etc... the original doctrinal shape of Christianity was given by Constantine, who was not Christian, in 325 AD (Concil of Nicea). for the process of Trinity belief:
"AT THIS point you might ask: 'If the Trinity is not a Biblical teaching, how did it become a doctrine of Christendom?' Many think that it was formulated at the Council of Nicaea in 325 C.E.
That is not totally correct, however. The Council of Nicaea did assert that Christ was of the same substance as God, which laid the groundwork for later Trinitarian theology. But it did not establish the Trinity, for at that council there was no mention of the holy spirit as the third person of a triune Godhead.
Constantine's Role at Nicaea
FOR many years, there had been much opposition on Biblical grounds to the developing idea that Jesus was God. To try to solve the dispute, Roman emperor Constantine summoned all bishops to Nicaea. About 300, a fraction of the total, actually attended.
Constantine was not a Christian. Supposedly, he converted later in life, but he was not baptized until he lay dying. Regarding him, Henry Chadwick says in The Early Church: "Constantine, like his father, worshipped the Unconquered Sun; . . . his conversion should not be interpreted as an inward experience of grace . . . It was a military matter. His comprehension of Christian doctrine was never very clear, but he was sure that victory in battle lay in the gift of the God of the Christians."
What role did this unbaptized emperor play at the Council of Nicaea? The Encyclopædia Britannica relates: "Constantine himself presided, actively guiding the discussions, and personally proposed . . . the crucial formula expressing the relation of Christ to God in the creed issued by the council, 'of one substance with the Father' . . . Overawed by the emperor, the bishops, with two exceptions only, signed the creed, many of them much against their inclination."
Hence, Constantine's role was crucial. After two months of furious religious debate, this pagan politician intervened and decided in favor of those who said that Jesus was God. But why? Certainly not because of any Biblical conviction. "Constantine had basically no understanding whatsoever of the questions that were being asked in Greek theology," says A Short History of Christian Doctrine. What he did understand was that religious division was a threat to his empire, and he wanted to solidify his domain.
None of the bishops at Nicaea promoted a Trinity, however. They decided only the nature of Jesus but not the role of the holy spirit. If a Trinity had been a clear Bible truth, should they not have proposed it at that time?
AFTER Nicaea, debates on the subject continued for decades. Those who believed that Jesus was not equal to God even came back into favor for a time. But later Emperor Theodosius decided against them. He established the creed of the Council of Nicaea as the standard for his realm and convened the Council of Constantinople in 381 C.E. to clarify the formula.
That council agreed to place the holy spirit on the same level as God and Christ. For the first time, Christendom's Trinity began to come into focus. Yet, even after the Council of Constantinople, the Trinity did not become a widely accepted creed. Many opposed it and thus brought on themselves violent persecution. It was only in later centuries that the Trinity was formulated into set creeds. The Encyclopedia Americana notes: "The full development of Trinitarianism took place in the West, in the Scholasticism of the Middle Ages, when an explanation was undertaken in terms of philosophy and psychology."THE Trinity was defined more fully in the Athanasian Creed. Athanasius was a clergyman who supported Constantine at Nicaea. The creed that bears his name declares: "We worship one God in Trinity . . . The Father is God, the Son is God, and the Holy Ghost is God; and yet they are not three gods, but one God."
Well actually there are many things to write about it but this may help to understand the process, which was done totally by the initiatives of humankind, not of God.
Regards.