I don't see where the Catholic/Orthodox religion was EVER the only game in town...No doubt they always tried to be the only game in town but it appears that God always saved a remnant somewhere to carry on...
Some Protestant theologizing is so extreme as to say that the gospel lived only in these splinter congregations and that there was nothing at all in the mainstream.
They can have their own opinions about bygones, but we CAN observe these congregations today and the fact that some of the worshipers ARE unmistakably Christian. They have theological errors but again it isn’t good theology that saves, it’s Christ.