At first Constantine, like his father, in the spirit of the Neo-Platonic syncretism of dying heathendom, reverenced all the gods as mysterious powers; especially Apollo, the god of the sun, to whom in the year 308 he presented munificent gifts. Nay, so late as the year 321 he enjoined regular consultation of the soothsayers5 in public misfortunes, according to ancient heathen usage; even later, he placed his new residence, Byzantium, under the protection of the God of the Martyrs and the heathen goddess of Fortune;6 and down to the end of his life he retained the title and the dignity of a Pontifex Maximus, or high-priest of the heathen hierarchy.7 His coins bore on the one side the letters of the name of Christ, on the other the figure of the Sun-god, and the inscription “Sol invictus.” Of course there inconsistencies may be referred also to policy and accommodation to the toleration edict of 313. Nor is it difficult to adduce parallels of persons who, in passing from Judaism to Christianity, or from Romanism to Protestantism, have so wavered between their old and their new position that they might be claimed by both. With his every victory, over his pagan rivals, Galerius, Maxentius, and Licinius, his personal leaning to Christianity and his confidence in the magic power of the sign of the cross increased; yet he did not formally renounce heathenism, and did not receive baptism until, in 337, he was laid upon the bed of death.
. 5 The haruspices, or interpreters of sacrifices, who foretold future events from the entrails of victims.
. 6 According to Eusebius (Vit. Const. l. iii. c. 48) he dedicated Constantinople to “the God of the martyrs,”
but, according to Zosimus (Hist. ii. c. 31), to two female deities, probably Mary and Fortuna. Subsequently the city stood under the special protection of the Virgin Mary.
. 7 His successors also did the same, down to Gratian, 375, who renounced the title, then become quite empty.
Schraff says here that the title Pontifex is “quite empty.” We should all remember that this title was the title given the pagan priests of the mystery religions. However after the death of Attalus III the last king of Pergamum, the title was left to Rome.
“The pontiffs were chosen among the most illustrious of the senators; and the office of Supreme Pontiff was constantly exercised by the emperors themselves. They knew and valued the advantages of religion, as it is connected with civil government.” The Decline and Fall of the Roman Empire by Edward Gibbon Vol. 1 page 56
Maybe it was no empty to the Roman emperors, however in 378 just two years later the Pope took up this pagan title of Pontifex, and has held it ever since.