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.
Contrition
A central concept in much of Christianity, contrition is regarded as the first step, through Christ, towards reconciliation with God. It consists of repentance for all one's sins, a desire for God over sin, and faith in Christ's redemption on the cross and its sufficiency for salvation (see regeneration and "ordo salutis"). It is widely referred to throughout the Bible, e.g. Ezekiel 33:11, Psalms 6:7ff, Psalm 51:1–12, Luke 13:5, Luke 18:9–13, and the well-known parable of the prodigal son (Luke 15:11–32).