Source of Paul's Great Confidence
Even if four persons hear the same long story from one and the same person, they will probably record vastly different four stories.
Even if the four Gospel authors had heard the episodes of Christ Jesus from Peter or someone who had actually witnessed the mission of Christ Jesus, the four Gospels should have shown great diversities. But they look like one another very much. So, those authors did not write their versions of Gospels depending on their interview with Peter or someone who had actually witnessed the mission of Christ Jesus.
Therefore, there must have been a written source for those Gospels. And, this written source must be in Hebrew or Aramaic, since the very early Christians who were also Judaists had almost no passion to preach the Gospel to foreigners or gentiles.
And, the great confidence of Paul that he understood Christ Jesus as much as Peter and other direct disciples of Christ Jesus tells that Paul had some concrete proof or report on the holy mission of Christ Jesus that must have been the written source based on which the four Gospels were written later.
Therefore the four Gospels according to Mathew, Mark, Luke, and John must have been based on one written source Paul had. But translation work caused some differences among them. Anyway, therefore, the four Gospels were chosen as authentic books for the New Testament.
But who had given the written source to Paul and where? This is one of the most important issues to understand Chrisitanity.
Luk 2:6 And so it was, that, while they were there, the days were accomplished that she should be delivered.
Luk 2:7 And she brought forth her firstborn son, and wrapped him in swaddling clothes, and laid him in a manger; because there was no room for them in the inn.