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.
Christian views on sin
Venial sins are sins which do not meet the conditions for mortal sins. The act of committing a venial sin does not cut off the sinner from God's grace, as the sinner has not rejected God. However, venial sins do injure the relationship between the sinner and God, and as such, must be reconciled to God, either through the Sacrament of Reconciliation or receiving the Eucharist (after proper contrition fulfilled).