If it weren't for St Paul, the marginal Jewish sect of Christus followers would've been all but extinguished during the Judaean Revolt of 65-70 CE. Whatever remnants survived would have vanished with the rise of rabbinic Judaism. We would likely never have even heard of Jesus Christ, since there would be no writings to preserve his memory (you do recall that Paul's epistles were written before the Gospels, right?)
If it weren't for the triumph of Christianity in the Greco-Roman world, Muhammad would've never come up with his obviously borrowed creed. Even assuming that he had become a figure of any significance, it's highly implausible that he would've encountered a world as conducive to his conquest.
And if it weren't for Christendom, Europe would've never held itself together as a coherent 'Western civilization' through the Medieval Era, and even if it had Newton would've never been inspired to try to work out the "divine order" of nature, even assuming that science had progressed that far, which is unlikely in a world of ubiquitous slave labor (which is probably why Roman science stagnated). If Newton's genius had prevailed nonetheless to make its mark on history, it very likely his publications would've been on a much reduced level than what they were, considering the probable disarray of European civilization.
And that's assuming Gutenberg had found something else inspiring enough to merit a printing press, since there'd be no bibles..
St Paul should be first.
Assuming arguendo you are correct, and I stipulated that without St. Paul there would be no Christianity (whether something else would have come along that filled the cultural demand (you do think religions tend to emerge to fit the prevailing ethos rather than the reverse don't you?), or Arab resurgence, one can only speculate), you are an actual cause guy, and Hart I think was more a proximate cause guy. As a law school graduate, you know the difference.