FTS, sorry, I like to be provocative...it would be more transparently correct to speak of the syncretism which was forged under Constantine the Great as a fusion of contemporary “christian” beliefs. However I wanted to highlight the paradox that the doctrinal basis of these early sects of Christianity was then only recently and resolutely drawn from pagan belief... outstandingly so was the long pre-Christian history of the Christ figure himself. Even Constantine kept the Solar Invictus (unconquered Sun) image on his coinage up until about 320CE well after his purported conversion to Christianity. This was in fact a pivotal teaching of solar theology at the root of the Christ myth back in pagan antiquity, which had visible echoes in early Christian iconography.
Think of any doctrine or NT story; it is likely that it has a pagan or a Gnostic antecedent. Odd intrusions into the NT text such as “I am the true vine” and “...upon this rock, I will establish my church” are tangible demonstrations of very pointed and specific gestures to existing cults, to that of Dionysus in the first case and Mithras in the second. Exotic words offered as a sop to the holders of these essential non-Jesuist doctrines in the on-going process of Roman Catholic syncretism; working in the familiar Roman imperial political manner of 'inclusion and assimilation'. For the sake of retaining their temple authority, It became prudent for cult leaders to make the move towards the teachings of the Imperial Roman Church.
The fact is there is little authoritative literature if any on the subject and hence it is not on the general scholar’s radar. The whole episode of forced conformity had to be covered up and that could only be done effectively through Imperial agency to start with. Later with the dilution of Roman power after Theodosius the Church itself became so powerful it could lead the anti paganism drive.
Overall I still cannot but think that Constantine was ever playing the role of emperor. He had his religious advisers such as Hosius and he had his publicists such as Lactantius and Eusebius but religion was for him an instrument of state. His nearly life-long tolerance of Roman cults and his ineffectual and fumbling attempts at directing the bishops towards doctrinal unison showed his heart was not in the business of Jesus-Christianity, as did his deathbed baptism. It was some of his offspring who took up the Christian banner and ran with it.