Emperor Constantine became Saint Constantine after declaring Christianity to be the official religion in Constantinopolis. He literally patronized Christianity and later instructed that Christians and non-Christians should be united in observing the sun worship.
He died. Then at some point in the future some pope decided he performed miracles from the grave - has to be at least three i think - then that pope canonized him and declared him a saint. The pope being the Voice Of God Himself of course. No one but catholics ever 'witness' these supposed miracles of course.
Hi, Zhaaaai15! 1. There's more than one St. Constantine, but I think you mean, Constantine I, also called, Constantine the Great, who was the first Roman emperor to become Christian, and the son of St. Helena, who was Christian, and a Roman empress, until Constantine's father put her aside. As empress, she travelled to the Holy Land, with 10,000 slaves, give or take a few, and brought back tons of Christian souvenirs, including the many stairs Jesus Christ had to walk up to be judged by Pontius Pilate; and the wooden Cross Jesus was nailed to until he died called, The True Cross; but you didn't ask about her. However, I don't think her influence on her non-Christian son can be denied. 2. Constantine I lived from about 272 AD to about 337 AD. 3. There was no official, formal process whereby dead Catholic Christians were declared, by the Pope, to be definitely in Heaven, that is, Saints, back then, so neither Constantine I, nor his mother, Helena, was ever canonized, as we understand it today. 4. Constantine I, and his mother, may have been made Saints, by acclamation, which is true of many of the early Catholic Saints, which means that almost all of the Catholic Christians, when it happened, thought that they should be Saints. 5. Since there was no formal process, there was no formal investigation into their lives, so there is no report stating the reason(s) they were made Saints. 6. I do think the fact that Constantine I reportedly had a vision in a dream wherein Jesus said to him: in this sign, conquer; and the fact that Constantine I always carried that Christian sign (the chi-rho, google it) into battle with him after that, and was victorious, had a lot to do with him being made a Saint. The fact that he made Christianity the official state religion probably helped, too. 7. I hope this helps to fully answer your question. More questions? Please leave a comment. Thanks! Frank