Oh good Heavens yes.
Remember that Pelagianism (No original sin, Man can choose to be good without the will of the Divine), Nestiorianism (the hypostatic union between human and Divine in the person of Jesus Christ) and Arianism (there is no human nature of Jesus Christ, only Divine) were all promulgated by theologians who considered themselves good Catholics working to better the Church, yet were still condemned as heretical and purged relentlessly.
What LUTHER unleashed created a violent and bitter schism that has lasted nigh on 500 years with casualties
even occuring in the modern day. Back when it started in the 16th century there were decades-long wars of Reformation and Counter-reformation and Inquisition that tortured, maimed and killed hundreds of thousands.
Imagine asking a Bohemian Hussite if he considered himself a good son of the Bishop of Rome, or asking a Jesuit tutor of Ferdinand II if Martin Luther had made some good points when he pointed out the practice of paid indulgences led itself to corruption.
Pre-Vatican II and Cardinal Wojtyla's "ecumenical" efforts I'm reliably told that not only did you not consider a member of some other sect non-Christian, you thought they were positively
Satanic.
