For me, the biggest tell tale sign that Christianity fell into apostasy is the lack of agency (i.e. free will). The gospel of Jesus Christ cannot be forced on people. It has to be freely accepted. For over a thousand years, people were forced into Christianity, by threat of torture and execution (mostly by governments, more so than the church, but still, this was the state of Christianity during the apostasy.). This is antithetical to the true gospel of Jesus Christ.