If, in conversion, all sins were forgiven to us, in the past, in the present and in the future, it would be useless for Christ to ask us to pray: “Forgive us our debts, as we have forgiven our debtors,” which he said, for “if you forgive men for their transgressions, your Heavenly Father will forgive you too. But if you do not forgive men for their transgressions, your Father will not forgive your transgressions” (Matthew 6:12-15). Mortal sin is defined by St. Augustine (response to Faustus XXII. 27) as “Dictum vel factum vel concupitum contra legem éternam,” that is, something that is said, done or intended against eternal law, or a thought, word or act contrary to eternal law. It is a definition of sin, because it is a voluntary act. As it is a lack or deprivation, it can be defined as an aversion to God, our true final end, because of the preference of a changing good. The definition of St. Augustine is generally accepted by theologians and is above all a definition of real mortal sin.
It explains well the material and formal elements of sin. The words “dictum vel factum vel concupitum” refer to the material element of sin, a human act: “contra legem éternam,” the formal element. The act is evil because it transcends divine law. Saint Ambrose (De paradiso, viii) defines sin as an “excuse of divine law.” The definition of St. Augustine, viewed with rigour, that is, as a sin that diverts us from our true end, does not include venial sin, but to the extent that venial sin is at odds with divine law, although it does not distract us from our final end, it can be said that it is present in the definition in its present form. First, a definition of the sins of omission, the sins of omission may be included in the definition, because they imply a positive act (St. Thomas, I-II:71:5) and denial and confirmation are reduced to the same species. Sins contrary to man or natural law are also present, for what goes against human or natural law is also contrary to divine law, as much as every just human law is derived from divine law, and not only if it does not conform to divine law.
According to St. Thomas (II-II:153:4) “a vice of capital is what has a very desirable purpose, so that in his desire a man passes the burden of many sins, all of which are said to come from this vice as the main source.” It is not the gravity of the vice itself that makes it a capital, but the fact that it causes many other sins. These are listed by St. Thomas (I-II:84:4) as vanity (Pride), varialut, gourd, lust, sloth, jealousy, anger. St. Bonaventure (Brevil., III, ix) gives the same enumeration. The ancient writers had distinguished eight sins of capital: so St. Cyprien (From Death., iv); Cassian (Institute 5, lectures 5); Columbanus (“Instr. octo vitiis princip. in “Bibl.max.” Vet. patr. XII, 23); Alcuin (De virtut. et vitiis, xxvii sqq.). Number seven, however, had St.
Gregory the Great (Lib. mor. to Job. XXXI, xvii) and it was preserved by the leading theologians of the Middle Ages. Sometimes the Lord lets us fight against sin so as not to become selfish or to think that we do not need God`s grace in our lives. St. Paul famously says, “What I want to do, I don`t do. But what I hate, I do” (Romans 7:15). Humility is the starting point of holiness.