Constantine, probably. I don't remember who he conquered, but he claimed that he did so from seeing the sign of the cross in the sky and its accompanying statement, "by this sign you will conquer".
He converted to Christianity, and as did the entire Empire. This occurred during the earlier part of the 4th century I believe. He founded the city of Constantinope which endured until conqueored by the Turks, around the time that Colombus discovered America.
Wotan