I had quoted many times and for many years that the majority of the New Testament can be reconstructed from the available church father quotes. I began to feel guilty that I had never confirmed this, until about two or three years ago, I actually did the research. I discovered that somewhere around 60-70% of the New Testament is quoted in the first four centuries. I was glad to learn that I had not been exaggerating.
We know from very early letters such as the First Letter of Clement of Rome, Didache, and the Epistle of Barnabas, that the New Testament texts were being used to teach and, presumably to preach, already by AD 100. I suggest you look up the Letter of Clement of Rome on your own to check this out. It is easy to find of the internet. You will see that he quotes the Old Testament and the New Testament with equal authority. This letter was written perhaps as early as AD 97.
Discussions about the nature of God began in earnest in the mid-second century. By the third century, Christian writers began to have many discussions about theology—considerably more that is normal for believers today. It was almost exactly AD 200 when Tertullian first used the Latin word “trinity.” His theology was essentially the same as that found in the Nicene Creed from the early fourth century. Nevertheless, as early as the third and even the second century, theologies and christologies which were later to be labeled as heretical began to circulate. There is a lot of information about this in my book The Christian Story: Finding the Church in Church History Vol I Available here: https://www.ipibooks.com/products/thechristianstory-volume1
In the end, church councils such as those at Nicaea, Ephesus and Chalcedon in the fourth and fifth centuries led to the publication of official church theologies which are in very good agreement with the content of the New Testament. You can read about all of this in my book, or you can look up the Nicene and/or the Chalcedonian creeds. They concluded that the Father, the Son and the Holy Spirit are unique persons, but of the identical substance. Chalcedon established the Christology that most of us would agree with today, which is that Jesus Christ was fully and perfectly human and God. I believe that there is plenty of Scriptural evidence in accord with their conclusions. I do not ascribe to human creeds personally, but I have no problem with what these councils published as being, essentially, accurate to the Bible.
Of course, all of our current understanding relies on the reliability of the New Testament text. We have somewhere close to 6000 ancient Greek texts, dozens from the first four centuries. That and the church father quotes allows us to produce a Greek text which is at least 99.5% accurate, putting no important Christian teaching in doubt. This is what the evidence shows, which allows us to judge for ourselves what the apostles and Jesus taught.