The man who propagated first Christianity outside Palestine was Paul

From: "The Bible came from Arabia" of Kamal Salibi 

Today, many scholars take the view that the Gospel writers acquitted Pilate of responsibility for the death of Jesus in order to make Christianity more palatable to the Roman Gentiles. But in fact the Gospels largely represent the point of view of the Hebraic party among the apostles. 

The man who first began to encourage the propagation of Christianity among the Gentiles, including the Romans, was Paul, who had nothing to do with the writing of the Gospels. Furthermore, the Jewish historian Josephus, writing three or four decades after the death of Jesus, endorsed what is written in the Gospels and by Paul about the Jewish responsibility for his execution. He wrote in the Antiquities (18:3:3): 'Pilate, at the suggestion of the principal men amongst us . . . condemned him to the cross.' So much, or perhaps slightly more, can be deduced about the historical Jesus from the Gospels, with the help of the little other material available; and Paul recognized this same historical Jesus as being his Christ. As already noted, Paul had probably never met this Jesus in life. What remains highly significant, however, is that when Paul decided to become a follower and apostle of Jesus, he did not go to find information about him in Jerusalem, among his original disciples who still lived in the city, as one would have expected. Oddly enough, and by his own sworn testimony ('What I write is true! God knows I am not lying!', Galatians 1:20), he went directly to Arabia instead. The first-hand information he needed about Jesus  either as a man, or as the promised Jewish Messiah  was to be found in Arabia rather than in Jerusalem. This means that the historical Jesus had some connection with Arabia. He was either a man who came from there, or the representative of an Israelite religious or political movement whose original centre was Arabian. Perhaps he was both. Paul's visit to Arabia is a historical fact. Yet the book of Acts, representing the predelictions of the Jerusalem apostles whom Paul never consulted about Jesus, made a point of ignoring it. This suggests that there was a great secret involved.