|Constantine and his mother, Helen as Saints|
The Catholic Encyclopedia (via New Advent) gives this account of his death:
Constantine himself preferred the company of Christian bishops to that of pagan priests. The emperor frequently invited the bishops to court, gave them the use of the imperial postal service, invited them to his table, called them his brothers, and when they had suffered for the Faith, kissed their scars. ...When at last he felt the approach of death he received baptism, declaring to the bishops who had assembled around him that, after the example of Christ, he had desired to receive the saving seal in the Jordan, but that God had ordained otherwise, and he would no longer delay baptism. Laying aside the purple, the emperor, in the white robe of a neophyte, peacefully and almost joyfully awaited the end.
Constantine was certainly THE MOST influential force in Western Christianity in the spread of the Gospel if you ask me. Moreso than Paul, even.
This Emperor declared that Christianity would be the official state religion and began to slowly but surely practice that faith. It's one thing to make a decree but when one is actually an adherent to that faith, that was a big deal. Simply, the Emperor was seen as a god among gods. And this was a god who declared that God was one and God was triune. Constantine actually sat in on the writing of the Nicene Creed, by the way. Aw, heck, who'm I kidding? He CALLED the Nicene Council together.
So why is he not a saint?
Although the Orthodox Church does recognize him as a saint, we Roman Catholics do not. The "saint-making" process is a bit different in the two faiths.
Despite his almost textbook "deathbed conversion", Constantine was a nasty piece of work when it came to dealing with his enemies and those enemies of Rome. He also had his wife and a son executed. Horribly.
Those factor in when one is being judged to be a saint. I think that sort of worked against any cause.
So, while he wasn't saintly, he is the reason we are Roman Catholic, by and large.
As saints are remembered by the day of their deaths as the day they enter Heaven, we wish Constantine a Happy Un-Saint's Day. Although, Constantine is not an official "saint name", wouldn't it be cool to use it for your Confirmation name? Then you could share it with this guy:
Aw, come on. YOU KNEW there would be a comic reference in this. You knew it!