Here's how I read the history:
Mediterranean and Near-Eastern mystical religions combined with Indian mystical religions, and spawned gnostic groups of varying stripes, the strongest of which emerged was Christianity which in most of its manifestations was gnostic in nature, save those furthest removed from the source, some of which were adopted by the Romans for political reasons.
Why? The Roman empire had reached its zenith and was beginning a decline. Salt (from which we derive the world salary, as salt was used as a primary currency, especially for paying the legions) was becoming less plentiful, the soil was declining in fertility due to unsustainable practices, deforestation was restricting the flow of timber and one of the causes of the agricultural issues, the social system had built up large inefficiencies in the form of the latifundia, the money supply was constricting due to the poor monetary policies of those who served as its bankers and financial legislators.
Enemies that had once feared the might of Rome became emboldened, and those who could see knew that the days of Roman military & political power were numbered.
Now, in the intervening time, the Romans had at various times suppressed the Christian and other religious groups that appeared within it. Christianity had secured itself a peculiar foothold, preaching in the catacombs and such, a literally underground force which commanded tremendous, secret zeal within Roman society.
Rome felt it, as their own secret police (and such) apparatus was constantly ferreting about at the edges; it recognized that Christianity was a powerful force for zealous obedience, and dreamt of harnessing it to their own ends.
They cherrypicked amongst the various Christian groups for the teachings most amenable to their interests, containing strong elements of guilt, control, confession, obedience to a socio-religious structure and its head as the infallible authority, the anti - (surrogate for) christ.
Missionaries carried this weapon deep into enemy territory, spreading it like a disease.
And so Rome fell to the sword of the Goths, but it was too late - they had been infected with the Roman strain of Christianity, and before too long, the Catholic Church wielded more absolute power in Europe than ever the Roman senate or imperial seat. The Pope was for all intents and purposes the Caesar, and he demanded both that rendered to Caesar and that rendered to God.
In order to consolidate their hold, they had to eliminate every competing strain. And so they stamped out the rest - Manichaeism, Mandaenism, gnosticism, pagan religions all around.
And for a millenia the RCC ruled Europe with an iron fist, with some competition and resistance from the landed nobility gathering momentum towards the end, until eventually under their partial protection, Greek pagan ideas conserved and expanded upon by the Persians came to Italy by the sea trade, fertilizing European minds ripe for change, and fostering the spirit of the Enlightenment to follow.
Secular Humanism then defanged the excesses of Christianity, and paved the road for new elites to dominate through trade and finance, areas which the Catholic Church itself had once dominated, leading to overall the potential for a freer, more materially well off, better educated and more rational society capable of returning itself to more valid forms of spiritual philosophy.
