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.
Mortal sin
Some mortal sins cause automatic excommunication by the very deed itself, for example renunciation of faith and religion, known as apostasy, desecration of the Eucharistic species, and "a completed abortion". Those mortal sins are so serious that the Church through law has made them crimes. The Church forbids the excommunicated from receiving any sacrament (not just the Eucharist) and also severely restricts the person's participation in other Church liturgical acts and offices. A repentant excommunicated person may talk to a priest, usually in a confessional, about their excommunication to arrange for the remission. Remission cannot be denied to someone who has truly repented their actions and has also made suitable reparation for damages and scandal or at least has seriously promised to do so. However, even if excommunicated, a Catholic who has not been juridically absolved is still, due to the irrevocable nature of baptism, a member of the Catholic Church, albeit their communion with the Christ and the Church is gravely impaired. "Perpetual penalties cannot be imposed or declared by decree." However, "the following are expiatory penalties which can affect an offender either perpetually..."