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:
Lust
Adultery: One of the main forms of lust seen frequently during the Middle Ages was the sin of adultery. The sin of adultery occurs when a person is unfaithful to his or her spouse, hence "invading of a bed not one's own," (Pg.235). Adultery is a special kind of ugliness and many difficulties arise from it. When a man enters the bed of a married woman it not only is a sin, but it "wrongs the offspring," because the woman now calls into question the legitimacy of children. (Pg.235). If a wife has committed adultery before, then, her husband will question if all his wife's children are his offspring. 
Simple fornication: Simple fornication is having sex with one's wife for enjoyment rather than for bearing children. Fornication is also sex between two unmarried people, which is also a mortal sin. Aquinas says, "fornication is a deadly crime," (Pg.213). Fornication is a mortal sin, but as Aquinas notes, "Pope Gregory treated sins of the flesh as less grievous than those of the spirit" (Pg. 217). Fornication was a grave sin such as that against property. Fornication, however, is not as grave as a sin directly against God and human life; therefore, murder is much worse than fornication. Property in this case means that a daughter is the property of her father, and if you do wrong to her, you then do wrong to him; therefore seducing a virgin or seeking pleasure from an unmarried woman is an invasion of a father's property.