Constantine the Great and his mother Helena are saints and we celebrate them on May 21. Most Orthodox churches also venerate them as saints. The Roman Catholic Church also regards them as saints. In southern Italy and some parts of Spain there are churches dedicated to St. Constantine and Helena. There is some disagreement about the time and place of his baptism. Most churches hold the view that he was baptized on his deathbed in AD 337 in Nicomedia, not far from Constantinople. It is very interesting that the Coptic Church interprets that St. Constantine was baptized by St. Pope Sylvester in Rome sometime after the persecution of the Christians ceased (after AD 313). I hope I have answered you at least in part.