As a result, during those years broad discussion about Christian values and theology was impossible. A small number of documents were in circulation, principally the four gospels we have now and some other new testament letters and writings. Principally Christians practised baptism and the rite of the Eucharist (now also called Holy Communion and the Mass). With the conversion to Christianity of Constantine the Great in the early 300s, Christianity became not only legal but also the state religion of the Roman Empire. Open debate sprang up everywhere but with very little common consensus.
The biggest question to be answered was who was the Christian God? The Christian writings of day were not totally clear about it. In fact the status of the Hebrew writings was not agreed upon either, meaning that even the God of the Jews might not be the Christian God!
Constantine called a council of all of the bishops who had survived his predecessor's anti-Christian reign of terror (Diocletian). They met to decide on what Christianity stood for at a small, insignificant town (even back then) called Nicea.
The big issue turned out to one of Christology, that is who and what is the Christ of the Christians. The Jewish God was accepted as the Christian God and the bishops accepted that Christianity was monotheistic. So what role did Christ play as the Son of God? What did the term "Son of God" or "Son of Man" mean? What did the Jews mean when they used the term "Messiah" from the time of Moses onwards.
The issue was divisive and is still not fully resolved.
One group lead by Arius believed the God created all things including the Logos (Greek for Word) who was born as Jesus Christ. The other group lead by Athenatius argued that the Eucharist was meaningless if the Christ wasn't really God and really human. Understanding Athenatius' Christology is a major effort in its own right. Understanding Arius's point is much simpler.
A review of Chapter 7 of the book of Wisdom or reading the beginning of the Old Testament makes it really plain that the Logos and the world were created: the Logos first and then the world through the Logos. This makes Christ, the Logos made human, God's messenger (or chief angel). In many ways this makes more sense of the Old Testament prophesies as the Jews understood them: they expected their messiah to be appointed by God, sent if you like, and lead them to eternal victory, both spiritually and physically. They expected a priest-king like David but without all of his faults and failings.
This is all very well, but it doesn't explain the role of Christ's crucifixion, resurrection and the institution of the Eucharist in the salvation of humanity. And everyone was sure that humanity had been saved.
The trouble is, as it says in the New Testament, only God can forgive sins. If the world wasn't made whole and humanity reconciled in Christ with God, what are the Christians on about? Athenatius wove a skilled theology, that left God as the creator, the prime mover of the Universe but made the Logos and the Holy Spirit "begotten and not created, being of one substance with the Father".
Thus Christianity, which has been since its beginning primarily a doing religion (with some hiccoughs after the collapse of the Roman Empire), has a God who did something for the forgiveness of sins. The theology of the Trinity is complex, but that is what you get if you have a God who messes in its creation to make things whole again.