In the late classical era and then on into the medieval period, there were huge transfers of wealth to the church. How did this happen? It wasn't as if the church suddenly revolutionized ancient warfare and took everything that it wanted by force.
On the contrary, wealth could be seen to "collapse" into the church. Large donors - both living and dead - would give huge grants of land to the church, and resources in the form of donations and labor would steadily pour in. Cathedrals and chapels themselves would become protected sanctuaries for endangered political subjects and by and large the sanctuaries were respected, though people would often be physically dragged out of the churches so the bad deeds could be done outside and out of protection.
The church would eventually become more integrated into the state, especially after Constantine adopted Christianity and successive rulers continued to formalized its official doctrines, and then the church itself became a fuller inversion of its genesis as it became the state and commanded military power.
But in that early transition period, Christian ideology was a force in itself: through the pronouncements and behaviors of its prophets and then adherents and religious and political leaders, it described a proper way to think and perceive and to be in a fallen world.
Through the material convulsions and the ensuing collapses of political and social legitimacies, the collapsing populations themselves had to become Christian after it became the dominant religious worldview in the ancient world, and the resources followed and flowed in.
Terrible catastrophe was embedded in the very DNA of the movement: the guilt and hatred of the material world, the self hatred, the rejection of wealth and standing, the apocalyptic visions, and so on. A lot of the ancient world collapsed into the Christian one.
