In various ways, what the church became after the NT era is of less importance, than what its foundational NT documents say it should be. There has always been a gap between Christianity on the ground, and the beliefs and behavior and praxis called for in Scripture. No one, I think, would debate this. Nor do I think there is much debate that Christianity became increasingly more like other established religions as time went on. It began to have buildings, it began to have priests, it began to have pilgrimages, it began to have multiple rituals, it began to have a cult of relics and a cult of the saints which in various ways adopted and adapted some things from the cult of the dead and the magical that characterized popular or folk Greco-Roman religion. RamsayMacMullen has demonstrated the connections and influence and similarities rather well in his book The Second Church. These things are pretty undeniable and they are the source of claims being made about ‘pagan Christianity’ in the post-NT era. Lest however we get ahead of ourselves. It is well to review the discussion by Johnson of Christians’ ways of being religious in the 2nd and 3rd centuries A.D. I would suggest, to sum up in advance, that the less ‘Jewish’ Christianity became in its makeup and also in its ways of thinking, and the less eschatological it became, the more susceptible it became to influence from pagan ways of being religious. Nevertheless, this did not lead the church to abandon a Jewish symbol system and narrative thought world, as embodied in both its foundational first century Christian documents and in the OT (in this case the LXX). It did not cause it to abandon either monotheism or a high ethical standard in things like sexual ethics either. Indeed, it was more apt to go in the other direction towards asceticism, even extreme asceticism in some cases. The idolatry and immorality banned by the Apostolic decree and the teachings of Paul and other early Jewish Christian thinkers was not reneged on, at least in the second century A.D., and we only see the beginnings of the cult of the saints and martyrs (including Mary as a saint) as a possible compromise of monotheism in the late second third, and early fourth centuries. Constantine did however fan the flames of such tendencies in various ways with his public endorsement of Christianity. So again, I would suggest even in the second and third centuries when Christianity was not a religio licita, and was persecuted, prosecuted, and some Christians were executed, it is not really correct to call Christianity another Greco-Roman religion, even at heart in its religious sensibilities, to judge from the documents produced during this period. Gnosticism was another matter, and as the first full-blown Christian ‘heresy’ it deserves to be treated separately from apostolic Christianity which went back to the movement’s founders.
In the first of his chapters on Religiousness A in the second and third century churches, Johnson once again chooses to focus on those documents and persons he takes to be representative of the period (Ignatius of Antioch, Polycarp, the Apocryphal Acts of the Apostles (including the Acts of Paul and Thecla) and the Apocryphal Infancy Gospels. He deals as well with Montanism and martyrs in Chapter 12. What he concludes is “it was not possible for Christians to partcipate in the regular round of ‘idolatrous’ public worship, and their own rituals were as yet largely undeveloped. The focus for this religious sensibility therefore became the holy person or saint through whom the divine dynamis worked and access to whom yielded benefits for others… those who bear witness to Christ in their violent death demonstrate the working of the same resurrection power in their triumph over imperial power, demonic power, and their own human weakness.” (p. 192). Here I think Johnson is largely correct. The more Gentile the church became, and the less atuned it was to its Jewish roots and theology these sorts of orientations became more possible, indeed anti-Semitic Gnosticism with its radical dualism about matter and spirit became possible. At the same time, Christianity was recognizably other, not seen as a legitimate Greco-Roman religion, and persecuted accordingly.