 Scholars debate whether Constantine adopted his mother St. Helena 's Christianity in his youth , or whether he adopted it gradually over the course of his life . Constantine would retain the title of pontifex maximus until his death , a title emperors bore as heads of the pagan priesthood , as would his Christian successors on to Gratian ( r . 375 – 383 ) . According to Christian writers , Constantine was over 40 when he finally declared himself a Christian , writing to Christians to make clear that he believed he owed his successes to the protection of the Christian High God alone . Throughout his rule , Constantine supported the Church financially , built basilicas , granted privileges to clergy ( e.g. exemption from certain taxes ) , promoted Christians to high office , and returned property confiscated during the Diocletianic persecution . His most famous building projects include the Church of the Holy Sepulchre , and Old Saint Peter 's Basilica . 
