Emperor Constantine was exposed to Christianity by his mother, Helena, but he was over 42 when he finally declared himself a Christian.[9] Writing to Christians, Constantine made clear that he believed his successes were owed to the protection of that High God alone.[10]
When Roman Emperor Constantine the Great (reigned 306–337) ruled Rome, Christianity became the dominant religion of the Roman Empire.
http://en.wikipedia.org/wiki/Constantine_I_and_Christianity
