What happens when Christianity is not the empire, and what happens when it is:
Before Constantine, one knew as a fact of everyday experience that there was a believing Christian community but one had to "take it on faith" that God was governing history. After Constantine, one had to believe without seeing that there was a community of believers, within the larger nominally Christian mass, but one knew for a fact that God was in control of history.
On living as a Christian:
What would happen if everyone did it? If everyone gave their wealth away what would we do for capital? If everyone loved their enemies who would ward off the Communists? This argument could be met on other levels, but here our only point is to observe that such reasoning would have been preposterous in the early church and remains ludicrous wherever committed Christians accept realistically their minority status. Far more fitting than "What if everybody did it" would be its inverse, "What if nobody else acted like a Christian, but we did?"