While that may be true dealing with the conversion of peter. Paul converted non-Jews, and he had large amounts of success. Also, I would argue, you are overestimating the size of the diaspora population, because so many where killed by Rome.
The earliest sources of Paul of Tarsus are from the New Testament, partially authored by himself, which makes all the information we have on him and his achievements highly partisan to say the least. Whatever the number of converts he made in his lifetime, the peoples he alledgedly preached to, such as the Hellenes and the Galatians, were a proud and highly sophisticated people. They would have been highly faithful and confident in their Gods they worshipped just as the Jews and Christians were faithful in their god. I don't think they just merely crowded around this travel worn stranger and meekly listened to him preach without taking his words with a pinch of salt. At worst, many of those that heard him preach might have been highly offended, not only that this stranger was quite conceited to believe that he knew better than them of the world, but that he would openly insult their gods or deny their existence.
Also, from the writings of Celsus, who was believed to be a platonist, the Christian groups had always maintained themselves as a secretive sect. He said that they would find new followers from amongst the criminal underworld, runaway slaves, the homeless, and even a few rebellious adolescents. It was convenient for them to recruit people from the lower order of society, the destitute and the vilified, giving them shelter and food. Basically using their kindness as a way to manipulate their gratitude and turn them into faithful followers. With means such as these, it might be easy to convert a few people.
Then also, I have talked with a number of professors who have argued that Plato would of likely been a early Christian had he lived in the time of Christ. I would argue, that they are looking at his metaphysics and not his political philosophy, but I am sure there were elements which also agreed with the Republic. Well, that is my last statement on this topic.