It seems amazing that the entirety of Europe was conviced to give up gods they had worshiped for generations and convert to Christianity so rapidly. What were missionaries saying that was so persuasive? What did Christianity offer them that other religions didn't?

by gorgagon
Dinocrocodile

There's a lot of good answers on this already and I recommend you check out these while you wait:

Assuming an atheistic perspective, what factors gave rise to the widespread adoption of Christianity in ancient Rome prior to it being made the official religion of the empire? by /u/Zosim

How significant was Constantine's conversion to Christianity? and Why did Christianity, and not another contemporaneous cult, spread so rapidly within the Roman Empire? by /u/talondearg

Also this thread:

How late did Greco-Roman polytheism / paganism last as a religion after the Roman Empire became officially Christian, and where were the last areas it was practiced?