Eternal sin
In Christian hamartiology, eternal sins, unforgivable sins, or unpardonable sins are sins which will not be forgiven by God. One eternal or unforgivable sin (blasphemy against the Holy Spirit) is specified in several passages of the Synoptic Gospels, including , , and .
Lust
St. Thomas Aquinas defines the sin of Lust in questions 153 and 154 of his Summa Theologica. Aquinas says the sin of lust is of "voluptuous emotions," and makes the point that sexual pleasures, "unloosen the human spirit," and set aside right reason (Pg.191). Aquinas restricts lust's subject matter to physical desires specifically arising from sexual acts, but Aquinas does not assume all sex-acts are sinful. Sex is not a sin in marriage, because sex is the only way for humans to reproduce. If sex is used naturally and the end purpose is reproduction there is no sin. Aquinas says, "if the end be good and if what is done is well-adapted to that, then no sin is present," (Pg.193). However, sex simply for the sake of pleasure is lustful and therefore, a sin. A man who uses his body for lechery wrongs the Lord.