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.
Sacrament of Penance
Practically all writings of that period, for instance "The Shepherd of Hermas", "Didache" or "Letters of St. Ignace of Antioch", show that grave sins were not rare among Christians. Cyrille Vogel collected a list of twelve major sins named in the New Testament and the Apostolic Fathers' writings. They are all various transgressions of the Ten Commandments: