Constantine adopted Christianity as the official religion of the empire.

That kept the empire alive for a couple more centuries and gave the Byzantines another millennia of time to fuck around. Christianity was central to Constantine’s continued control of the military, which had stopped being an elite profession of the nobility and transitioned into a proletarian wage labor position.

Absent a strong socially cohesive force to keep his armed forces loyal, the Roman state would have disintegrated a lot sooner.