Christianity just took over and mostly by force. You had to be publicly Christian (of some denomination) otherwise you were killed, tortured, imprisoned, etc. it wasn’t the fault of the religion as a religion can’t actively do anything but it was absolutely the fault of those who practiced at the time. Theocracies were a massive part of that.
Even if you Ignore things like the crusades, conversion because of Christian missionaries played a part. Like the stories of St Patrick bringing Christianity to Ireland and converting people from the Druid tradition. Whether you think missionaries converting groups is a good or bad thing, we have to acknowledge it IS true Christianity had a big part to play in people leaving their other beliefs behind.