What did the Early Christians Believe About the Trinity Before the Gospel of John? 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.&quot; (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.