Legend has it that he saw a cross in the sky and heard the words,"By this sign conquer."
Again, legend. But I think that Constantine saw that he was not going to wipe out Christianity, and decided to go with the flow. Just because he made Christianity the official state religion doesn't mean that he really believed and or practiced it. I have read that he continued in paganism though the Empire changed to Christianity. He was Emperor remember? The Army and Senate would probably go along to save their butts, and what did they care? Pagans incorporated new gods as they went along, and even the Roman gods were a mix of Greek gods and others they had come across in their conquests. Also, his word was law. If he said that Christianity was now the state religion, who was going to protest? I think the persecutions had probably stopped by then, but legal status had yet to be obtained. I'll have to go look it up to be sure.