As the classical world collapsed, Christianity followed the armies everywhere and the church became a secular power in its own right. Emperors and barbarians alike converted to Christianity for political reasons since the majority of the subject populations in their new territorities were Christians of one type or another (there were dozens of different sects within Christianity, later to be called heresies and violently suppressed). These secular rulers got baptized, then went on with their raping, pillaging and plundering with the full blessings of the church. There was no contradiction in this, however. The primitive Christianity of small, scattered congregations living a communal existence of peaceful brotherhood had turned into a wealthy, bloodthirsty monster.
Unleashed with the military power to impose its dogma, the church proved to be as bloodthirsty and fanatical as the Khmer Rouge, launching a massive assault on pagan beliefs, institutions and sacred places. Rampaging mobs took a break from slaughtering each other over doctrinal differences to destroy pagan temples, books, tombs and statues. Polytheistic paganism, the most tolerant of religious systems, was methodically suppressed, its adherents hunted down and killed or forcibly converted in a campaign of terror that went far beyond anything the Roman emperors were supposed to have done to the early church. Gibbon describes this mind-boggling campaign of official persecution in great detail in The Decline and Fall of the Roman Empire.
Late-classical and Medieval Christianity had a lot in common with Twentieth Century Communism, launching purges against its ideological enemies. In its fanatical zeal to impose doctrinal purity (and secure its secular control), the church turned against what it called "heretical sects" like the Cathars and Arians. Charlemagne, for instance, presented his subjects with the choice of conversion or death and murdered thousands in the process. During the years when official doctrine was still being formulated, crowds fought pitched battles in the streets over questions like the divinity of Christ and the true nature of the Trinity. Gibbon describes these insane riots in the Decline and Fall and blames the collapse of the Roman Empire on "barbarians and Christianity." Christians who blame the fall of Rome on "immorality" conveniently forget that Rome was the capitol city of a nominally Christian empire when it fell to the Visigoths.
Most of this history has been long forgotten now. Europe and the United States have been Christian for centuries--in the sense that Christianity is the dominant religion--and like all victorious ideologies, Christianity has become normalized and the old paganism is dismissed as "superstition"--the fate of all defeated religions. The true, bloody nature of the Christianizing of the West has been suppressed or glossed over and blamed on "politics," and Christianity itself has splintered into hundreds of different, squabbling denominations. For the most part, the religion itself has degenerated into a form of vapid liberalism, a "cult of social work" that has been an easy target for Social Marxist infiltration. Paganism still survives in a marginal "New Age" form that has nothing in common with the original, but for all practical purposes, polytheism, the true religion of the West, is dead.
Personally, I don't see this as progress.