Agreed there does not have to be a "true religion" from which an apostacy could occur. There is no such thing as a true religion in the first place, such an impression is surely a subjective one by those brought together under the spell of their own particular group's tradition and being emotionally bonded to it.
Neither is there evidence for a "great apostacy" with the Roman Church corrupting Christianity. The Romans were polytheist and had a tolerant attitude, they promoted piety, an outward, visible fidelity to the gods which they held was more important than to which one you venerated.
To the Romans, the various sects making up Christianity were a political annoyance because they were intolerant of others beliefs, they were monotheist, pacifist and had an interior belief system based on faith and obedience to God even when it was at odds with the state. Early Christians even relished persecution and martyrdom which in turn attracted other religious idealists to bolster the movement. It is therefore more of a surprise that the Christian Church, which was drawn from all the older religions especially Mithraism should become the dominant one by the end of the fourth century. The last Mithraic Pope Praetextatus, was offered the position of first Roman Pope in the state role of Ponifex maximus (great bridgemaker) but he declined. Of course the RC church later fabricated their links back to Peter to establish a spiritual pedigree springing from him.
Christianity was cultic from the start and Constantine for a number of reasons, not the least of which was because his beloved mother Helen was a believer, pressed for a unification of all religious belief under the one Imperial umbrella. He succeeded in creating legal toleration of Christianity in 313, (under Diocletion it had been banned), the Bishops had their heads banged together in 325CE at Council of Nicene (at what is today Isnik Turkey) and unified a universal (catholic) doctrine which was implemented only as late as 380 CE.
This is a big subject but necessary to study if we are to understand early Christianity. In this regard, the Bible does not help us! Like all cult outpourings it is entirely one sided. We need the voice of the historian archaeologist and sociologist to disentangle myth from the facts.
Rome declined and fell, but Roman Christianity took on the mantle of secular power and took Europe and Western Asia into centuries of the Dark Ages.
Yale does a lot of good You tube lectures on this period.