Why did Christianity grow and become so popular?

In class today we had a heated (i'd like to think so) argument about two different readings that had different ideas of why christianity grew and became popular.  My reading reasoned that it became popular because of the social ease and acceptance provided to all social classes.  The reading mostly touched on ideas about how the pagans were outcasting and accusing the christians of being atheist, but there were some points you could pull out from the text.  One of the major points being presented in the text was the idea of how christianity was such a open armed religion.  People who were young, not as wealthy, and questioning their beliefs favored it because it provided so many benefits and was a quite simple understanding.  It also was branched off of Judaism so people interested in the were assuming that christianity was the modernized version.  The idea of this religion grew so fast because of the ways in which the disciples enforced the idea of how you could tell your friends, family, neighbors, whoever about this religion and they could participate. The idea of passing through people was the number one way of getting something spread around, just like the black plague idea (except no constant deaths) In the other group it talked more about the idea of the beliefs being strong, like the actual idea of the religion was appealing to people. I think that that was a factor in the spreading of it, but it was not the full deal. Overall I think that people give into beliefs when it offers them so many appealing advantages. Why wouldn't you want to be in something simple and giving feelings of being within something greater.