Eternal sin
Thomas Aquinas lists, or has responded to, six sins that supposedly go against the Holy Spirit:
Thomas Aquinas explains that the unforgivability of blasphemy against the Holy Spirit means that it removes the entrance to these means of salvation; however, it cannot hinder God in taking away this obstacle by way of a miracle.
Caelestius
Caelestius also went much further than Pelagius in stating that the sin of Adam, as chronicled in the Biblical Book of Genesis had only harmed himself, and not all of humanity, as Augustine had taught in his writings and sermons (Pelagius has been accused of starting this teaching, but in fact he anathematized the opinions of Caelestius at the Synod of Lydda in 415 A.D., as well as in a letter to the pope shortly thereafter). In several books on the topic, Augustine also argued that Pelagius and Caelestius neglected to take God's divine grace into account. Augustine believed Adam's sin ("Fall") in the Garden of Eden had caused human beings to lose the ability to not sin. (""non posse non peccare"" in Latin) and therefore, all good deeds done by Christians come from them being enabled by God to do good. In contrast, Pelagius believed that God gave the power of free will to all men, not just Christians, in such a way that no one was forced into sin (Augustine even wrote certain remarks that seemed to imply that any such forced action would not in fact be sinful). Caelestius again appears to have gone beyond this and denied that Christian goodness is due to grace, on the grounds that this would imply that if any Christian sinned, it was because God's grace had failed; once again at the Synod of Lydda Pelagius anathematized this position (although he stated that he did not mean to indicate whether they were Caelestius' opinions or not).