I think Christianity today is extremely Johannine, meaning very dependent on the teachings of the Gospel of John. In my studies, I came across the fact that most Christian communities of the late first and early second centuries only had one gospel to listen to during meetings (not read, since few knew how to read). This raised a problem for me, since without the Gospel of John, the rest of the gospels and the Pauline epistles are not sufficient to teach the Trinity. Does this mean that the early Christians did not believe in the Trinity? I have been reading the other gospels and do not find the concept of Trinity being taught in them.
1) First to establish a date for the Gospel of John
Papyrus p52 puts the date in the first century. The traditional date is in the 90's, but these is a tendency to date the Gospel in the 70's before the fall of Jerusalem, but after Peter's death.
Here is more evidence for the authenticity of the Gospel of John. The two words for love in John 21:15 has long been debated. The second word that Peter used is the verb whose noun form means friend. Sometimes people bring up that Jesus most likely spoke these words in an Aramaic influenced Hebrew rather than Greek, followed by dismissing the difference. However, Samuel Olyan wrote in Friendship in the Hebrew Bible:
Although the biblical text has no word for “friendship,” there are a number of words for “friend.” Most common is rēaʿ and related nouns such as rēʿâ, raʿyâ, rēʿeh, and mērēaʿ, each apparently derived either from a root r ʿ h or a root r ʿʿ, both meaning something like “to associate with” or “to affiliate with,” suggesting a voluntary dimension to friendship." (Olyan, S. M. (2017). Friendship in the Hebrew Bible. (J. J. Collins, Ed.) (p. 4). New Haven; London: Yale University Press.)
What is interesting about this root is one of it's homonyms means to feed sheep, and is the verb used to translate such in John 21:15 in the Syriac (Aramaic) Peshitta and Hebrew translations of the New Testament. Thus, Jesus made a play on Peter's words. This play on words is too striking to be coincidence. Not only does it substantiate the authenticity of the account, but also the historicity of the resurrected Jesus.
Here's more evidence that Jesus uses play on wordsa:
... (Lk 16:11, my translation). This text exhibits a play on words in Aramaic, which was the language Jesus spoke at home. He says:
If you have not been amin [faithful] in the unrighteous mammon [your material possessions] the amuna [the truth] who will ja’min ith kun [entrust to you].
The root amn, which appears in the word amen, is used here four times. -- Bailey, K. E. (2008). Jesus through Middle Eastern Eyes: Cultural Studies in the Gospels (p. 379). Downers Grove, IL: IVP Academic.
It was a tool to help his disciples remember.
2) Christian belief before the Gospel of John
Paul's letter to the Galatians and 1 Corinthians, as well as Acts 15, show that theology among the church was not well established in the first century. In Galatian Paul struggled with those who taught Christians must keep the Law of Moses. This was settled in the Jerusalem conference in Acts 15. In 1 Corinthians Paul confronted divisions in the church at Corinth. In 1 Corinthians 15 he dealt with Greek philosophy of the spirit being set free from the body at death creeping into the church so that they did not accept the bodily resurrection. Many of the letters in the New Testament show the church's struggle for orthodoxy.
Part 1 established the authenticity of the teachings of Christ. The early church believed the teachings of Christ. There was variation of belief in the early church, but because John based his gospel strongly on the Christ's teachings, which he quoted extensively, there is no reason to believe the the Apostles and the core of the church didn't believe what John's gospel contains.
They believed the doctrines that were in the Gospel of John!
A head-on repost to the teachings (concerning the four gospels) of Dr Bart Ehrman would be "The Case for Jesus" by Dr Brant Pitre. Dr Pitre was a student under Dr Ehrman who came to see the absurdities of Ehrman's views.
I think you need to read the synoptic gospels more carefully. There is plenty in them to show that Jesus knew he was God manifest in the flesh, not least in his forgiving sinners all their sins.
When he was accused of blasphemy in various situations he never tried to reason with his accusers that they were misunderstanding him. If he was not claiming to be God in the flesh, he could have corrected the misunderstanding of his enemies and perhaps escaped crucifixion.
If you want more confidence in the gospels from a scholarly source then you must read "Jesus and the Eyewitnesses" by Dr Richard Bauckham: it doesn't get much more scholarly than that.
But give the synoptic gospels another read first, it may save you a lot of bother.
That when the Messiah comes he would be God is found in the OT - for instance Isaiah 9:6 "He shall be called... the mighty God"; "the Lord shall suddenly come to his temple" (Malachi 3:1) (the temple is God's, so the Lord spoken of is God). Jesus is called the Saviour, but "besides Jehovah there is no Saviour" (Isaiah 43:11).
And then also, the Letter to the Hebrews was written when the Temple was still operating as is shown by chapter 13 verse 11, we have an alter, whereof they have no right to eat which serve the tabernacle, which is in the present tense - they are still serving in the Temple in Jerusalem. He calls it a "tabernacle" to stress the temporary nature of the Temple.. its purpose is finished and its function is coming to an end. But clearly the Letter to the Hebrews was written before the Jewish Rebellion at the end of which the Temple was destroyed, never to be rebuilt.
With the date of when it was written in mind we can see that the early church had very clear teaching about the divine nature of Christ:
Christ is the heir of all things;
Christ is the brightness of the glory of God; a mere creature could not possibly be the brightness of the infinite God;
Christ upholds the whole Universe by the word of his power; no creature could do this;
Christ he sits down at the right hand of God the Father - compare this with Isaiah 42:8 my glory will I not give to another, nor my praise to graven images;
God the Father gives command to the angels: Let all the angels of God worship Christ. Let them angels proscuneo/ worship Christ as did the wise men from the East (Matt 2:11) and many others. But we are forbidden to worship any except God (Rev 22:9).
And finally, the Father speaking to the Son calls the Son "God", saying: Thy throne, O God, is for ever and ever.
One of the clearest statements of the Trinity is found, not in John's Gospel, but in Matthew's: Go therefore, and teach all nations, baptizing them in the name of the Father, the Son and the Holy Ghost: teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even to the end of the world. Amen (Matthew 28:19-20).
First, it is in the name (singular) of these three persons that we baptize; and, second, how can Jesus promise to be with you always even to the end of the world unless he is God, omnipresent?
There is plenty in the writings of Paul concerning the Trinity and the deity of Christ. Perhaps most famous is his statement God was manifest in the flesh (1 Timothy 3:16) which some argue was part of a doctrinal statement of faith of the early church. For a scholarly investigation of what are the original Greek words - whether it says "God" in 1 Tim 3:16 in the original - read John Burgon's 70 plus pages on the verse in "The Revision Revised".
Finally, I guess in reading the work of some "scholar" you say you came across a "fact" concerning the early church. If I were you, I would be more wary of these "facts" of scholars who know so much about the early church.
Luke's Gospel was written even earlier than 1st Timothy, as evidenced by comparing 1 Timothy 5:18 with Luke 10:7. Yet most, even scholars, would agree that Matthew's Gospel and Mark's Gospel were written before Luke.
This would really do best as a comment, but since nobody has approached it yet: early Christians were the first protestant Jews. I'm a Jewish convert to Catholicism.
The Jews in Jesus's time listened to the Talmudic scholars, who taught the Pentateuch literally, the rabbis, and the listeners thought "I like eating pork - it's yummy and cheap." And "nobody but nobody is going to tell my daughter that she's in a state of shame because she's nourished enough to menstruate," and other encouraging tidbits.
By the way, Jews are literate. They have always been literate - at least the men. Reading from the Torah (Pentateuch) is a rite of passage, and the men must go through it to marry, own property, etc. They are literate in Hebrew because that's the language of thr Torah. It may have been their second language behind Aramaic or even Greek, but that's a giant can of worms.
The Jews of Jesus's day were probably most similar to today's American/European Jews. They tried not to stand out, they gathered for Passover and ate matzohs, they prayed the Psalms, read from the Pentateuch and Prophets on a yearly cycle, and they celebrated the holidays from the Pentateuch (succot/booths). They learned prayers from antiquity, and they already believed in the Holy Spirit (the "spirit of God" from Genesis 1:2, literally ruach ehlohim). Ehliahu, Elijah the Prophet, still announces the coming of the Messiah (it's just not Jesus).
Forgive my lack of reference, Jesus was presented as the "Jewish messiah." It was probably several generations before those that sloughed Judaism aside for Christ laid their Jewish habits to rest. Of course, Christians took the entire Hebrew bible with them, so there may have been fewer differences - between Jews that stayed with Judaism versus those that followed Jesus - than we appreciate.
As for the gentile conversions, I don't really have any information on that.