Eternal sin
In Catholic teaching there are six sins that blaspheme against the Holy Spirit. They are: Several passages in the Bible are frequently interpreted as referring to the unforgivable sin:
Zoophilia
In Part II of his Summa Theologica, medieval philosopher Thomas Aquinas ranked various "unnatural vices" (sex acts resulting in "venereal pleasure" rather than procreation) by degrees of sinfulness, concluding that "the most grievous is the sin of bestiality." Some Christian theologians extend Matthew's view that even having thoughts of adultery is sinful to imply that thoughts of committing bestial acts are likewise sinful.