No Christians before the Fall of Jerusalem
Paul thought that the end of the (Jewish) world and the Last Judgment were really close.
Who influenced Paul so that Paul came to have this notion? It must be Christ Jesus.
From the beginning Christ Jesus was a kind of priest and missionary of Judaism. He did not work to launch a new religion but acted within the religious scope and paradigm of Judaism. So, when Christ Jesus preached that the Last Judgment was near, it must be interrupted in the context of Judaism.
And indeed, the Jewish-Roman war erupted in 66, which resulted in defeat of Jewish insurgents against the Roman Empire and destruction of Jerusalem. Eventually, Judaists were forced to leave Palestine and live in Diaspora. This incident could be really called the end of the world for Judaists and the Last Judgement on the Jewish people. In this context, the warning and prophecy by Jesus came true as Paul expected.
So, we had better think that there was no Christianity till the destruction of Jerusalem in the Jewish-Roman war that started in 66. Christ Jesus was a missionary of Judaism but not Christianity, since Christianity started 30 years after the Crucifixion of Christ Jesus. Accordingly, Christianity started after the martyrdom of Paul (and Peter) that happened before the Jewish Roman war.
As the Christian Bible includes the Old Testament, Christianity from the birth of Christ Jesus to the Jewish Roman war was actually Judaism before the Diaspora. Real Christianity started after the fulfillment of the prophecy by Christ Jesus and the foresight of Paul that the end of the world and the Last Judgment should happen soon, which actually occurred 30 years or one generation after the Crucifixion or the end of the mission of Christ Jesus.
Accordingly, the Gospels of Mark, Luke, and Mathew that must have been written before the Jewish-Roman war were rather holy books in the last period of Judaism before the Diaspora than the holy books in the earliest period of Christianity.
The Jewish-Roman war and the fall of Jerusalem must have a huge impact not only on Judaists in the first century but also on the early followers of Christ Jesus' teaching (though they believed that they belonged to Judaism). But this great shock is not reflected in the Gospels of Mark, Luke, and Mathew. Only the Gospel according to John reflected this radical impact. The followers of Christ Jesus' teaching could not rely on any merits or traditions of Judaism any more, since the Last Judgment on Judaists came true. Accordingly, John wrote a Gospel of his version based on this notion. That is why the Gospel according to John is separated from the synoptic Gospels according to Mark, Luke, and Mathew.
Conversely, the synoptic Gospels according to Mark, Luke, and Mathew were written before the Jewish-Roman war when Christianity was not yet established but followers of Christ Jesus' teaching and Christ Jesus Himself believed they were part of Judaists and followers of Judaism. However, the Gospel according to John which must have been written in awe in the aftermath of the destruction of Jerusalem in the Jewish-Roman war was the real first holy book of Christianity. In this sense, even the letters by Paul included in the New Testament belonged to the (Christian) era before real Christianity.
To try to feel the great magnitude of impact Judaists and followers of Christ Jesus' teaching felt when they saw the fall of Jerusalem in the Jewish-Roman war must be the first step to understand Christianity. Christ Jesus Himself, Paul, and early followers of Christ Jesus' teaching were all actually Judaists before the fall of Jerusalem.
Put simply, there were no Christians but only Judaists before the fall of Jerusalem in the Jewish-Roman war. Therefore, the Gospels according to Mark, Luke, and Mathew were written by Judaists but the Gospel according to John was written by a Christian.
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Luk 9:6 And they departed, and went through the towns, preaching the gospel, and healing every where.
Luk 9:7 Now Herod the tetrarch heard of all that was done by him: and he was perplexed, because that it was said of some, that John was risen from the dead;