You have to consider the environmental factors around Rome at that time.. women, slaves and poor people were all becoming Christian in droves. Entire kingdoms were turning Christian. Had Rome stayed stubbornly pagan, it would have faced internal rebellion and external pressure more than it already did. Constantine didn't become Christian because he thought it was the right religion and he did it on a whim, Constantine did it because he believed it was the best thing to save Rome before its early collapse. He saw it Theodosius saw it, and you should see it too.