There is a huge, complicated literature on the question of whether Constantine I ‘converted’ to the Christian faith. Sometimes the simplest argument is the best. Joseph Vogt of Tübingen says in the 1960s: “I prefer to trust Julian the Apostate’s bitter reproach to his uncle of having deserted the sun god, Helios.” That’s one line worth one thousand pages. QED.
(Copyright: Tobias-Bild Universitätsbibliothek Tübingen)
Source: J. Vogt, “Pagans and Christians in the Family of Constantine the Great”, The Conflict between Paganism and Christianity in the Fourth Century (Oxford–Warburg Studies), ed. A. Momigliano (Oxford: Clarendon Press, 1963), 40. (Modified for extraction.)