There were large scale social movements to believe in Isis, but she lost out. Jesus won because Christianity's characteristics gave it an evolutionary edge. Those were: greater missionary zeal, the Jewish diaspora network and a community of Jewish wannabees called the "God fearers", an ancient sacred book, a stronger emphasis on morality, greater emphasis on doctrine,hellfire,and more hierarchy. Admittedly, these don't include evidence or sense, but they aren't chopped liver. As for being trendy, that would have applied in the third century, but it's been awhile. I think I'd have to agree with neo on that.