Really? Mind sharing historical proof to support this?
Take the Iberian peninsula for examples. Between the fall of the Roman Empire, there was a mishmash of different versions of Christianity: Roman Catholicism, Aryanism, Priscillanism and Catharism. However after the muslim invasion conversions remained rare.
More generally, in the early european middle ages there were many offshoots of the church that were later repressed as heresies but would rapidly take hold of some areas if a leader decided to convert, but I can't think of any area of leaders who made up a whole new faith: they were all clearly based on Catholicism.
It's not true that the only cases of conversions are modern; rather the only variant faith who survived are modern, but there were others before.
