"The RCC priesthood could start offering sheep ..."
You should know better. Christ instructed the Church to preach everything he taught (Matt. 28:19–20) and promised the protection of the Holy Spirit to "guide you into all the truth" (John 16:13). That mandate and that promise guarantee the Church will never fall away from his teachings (Matt. 16:18, 1 Tim. 3:15)
He never said that anything calling itself a church was infallible. He, all scripture is His word, warns again and again of false teachers and false doctrines and Judisers(sp), those trying to reinvent and apply Jewish laws and traditions. He never warns about the scriptures though, thought He does warn that some twist the scriptures to fool you.
But ultimately, all we have to do is examine the fruits of the popes and we know them by their fruit. The main and most telling fruit is Marianism.