Sorry if this question was asked before but I could not see any post directly questioning the conversion of Constantine to Christianity. As a start, one might delve into what Christianity meant and how one could become a Christian at Constantine's time. How realiable are the accounts telling that Constantine converted to Christianity?
While some have called into question Constantine's religiosity and confessional identity throughout his life there is little doubt he was baptized before his death.
The baptism of Constantine the Great is recorded in The Life of Constantine by Eusebius of Caesarea. In the text he records Eusebius of Nicomedia performing the honour in the vicinity of Nicomedia after Constantine fell ill. Eusebius of Caesarea, The Life of Constantine trans. and ed. Averil Cameron and Stuart G. Hall (Oxford: Clarendon Press, 1999) IV, 61-64.