Many skeptics will say that most of the details about Jesus' life, particularly his miracles and resurrection, were added on later due to legend or crafted to make a viable religion out of their dead Messiah. But, ask yourselves, who was this new religion crafted for? Assuming for the moment that the resurrection was made up, the new Christians' message of a resurrected Jesus would not have appealed to Greeks or Romans. To them, our body was a cage, and the goal was for our souls to "fly away" (sound familiar - even some hymns have non-Christian influences...). No Greek or Roman would want a resurrection. For them, the body is crude and vulgar, and the soul craves freedom.
So, who's left? The Jews. Yet the idea of a crucified Messiah was appalling to the Jews. Their Messiah was a victorious ruler, not a dead-dog of a man hanging on a cross.
What, then, is the point of this new religion that focuses on a man come back to life? Why craft a new religion that makes no sense to or offends all of your potential converts? Oh, and preaching this new thing you just made up will get you beaten or killed by these people you've just upset. If you're going to make up a narrative about your Messiah, why that one?
No Jew would have made up a crucified Messiah, and no Greek or Roman would have wanted a resurrected one.
Thus the best explanation for the birth of Christianity, rather than being an invention, is that this dead-dog Jesus actually got up again a few days later and appeared to his followers. Imagine the shock of seeing your Rabbi walking toward you a couple of days after you just saw him killed. That's the sort of thing the one doesn't forget.
The rise of Christianity must be explained in a way that makes sense of the historical realities.