Ask Eusebius of Caesarea, and he won’t give a straight answer. Ask Jerome, and he knows it was the Arian Eusebius of Nicomedia. Ask anyone between the sixth and the sixteenth century, and they’ll tell you, with great assurance, Sylvester of Rome.
Hans Pohlsander (Emperor Constantine, 25-6) deftly summarizes the sources: “this strange tale enjoyed a long life and exerted considerable influence. It was accepted by the Liber Pontificalis (Book of the Popes), a collection of papal biographies first compiled c. 530. Later in the same century it underwent considerable embellishment, as happens so often in hagiography, in the Chronicon of Johannes Malalas: Constantine, it says, ‘was baptized by Sylvester, bishop of Rome, he himself and his mother Helena, and all his relatives and friends and a whole host of other Romans.’ The Chronicle of Theophanes Confessors, dating from c. 810-15, claims to know that Sylvester baptized both Constantine and his son Crispus in Rome and dismisses reports to the contrary as a forgery of the Arians . . . ; it seeks to prove its case by pointing to the ‘Baptistery of Constantine,’ meaning the Lateran Baptistery.”
Both the Previarum Romanum of 1569 and the Martyrologium Romanum of 1584 repeat the story. In 1588, Sixtus V brought an Egyptian obelisk to Rome Rome and inscribed it with “Constantine was baptized here.” As late as 1906, this view was still being defended.