Objection 1. It would seem that all sins are connected. For it is written (James 2:10): "Whosoever shall keep the whole Law, but offend in one point, is become guilty of all." Now to be guilty of transgressing all the precepts of Law, is the same as to commit all sins, because, as Ambrose says (De Parad. viii), "sin is a transgression of the Divine law, and disobedience of the heavenly commandments." Therefore whoever commits one sin is guilty of all.
Objection 2. Further, each sin banishes its opposite virtue. Now whoever lacks one virtue lacks them all, as was shown above (Question 65, Article 1). Therefore whoever commits one sin, is deprived of all the virtues. Therefore whoever commits one sin, is guilty of all sins.
Objection 3. Further, all virtues are connected, because they have a principle in common, as stated above (65, A1,2). Now as the virtues have a common principle, so have sins, because, as the love of God, which builds the city of God, is the beginning and root of all the virtues, so self-love, which builds the city of Babylon, is the root of all sins, as Augustine declares (De Civ. Dei xiv, 28). Therefore all vices and sins are also connected so that whoever has one, has them all.
On the contrary, Some vices are contrary to one another, as the Philosopher states (Ethic. ii, 8). But contraries cannot be together in the same subject. Therefore it is impossible for all sins and vices to be connected with one another.
I answer that, The intention of the man who acts according to virtue in pursuance of his reason, is different from the intention of the sinner in straying from the path of reason. For the intention of every man acting according to virtue is to follow the rule of reason, wherefore the intention of all the virtues is directed to the same end, so that all the virtues are connected together in the right reason of things to be done, viz. prudence, as stated above (Question 65, Article 1). But the intention of the sinner is not directed to the point of straying from the path of reason; rather is it directed to tend to some appetible good whence it derives its species. Now these goods, to which the sinner's intention is directed when departing from reason, are of various kinds, having no mutual connection; in fact they are sometimes contrary to one another. Since, therefore, vices and sins take their species from that to which they turn, it is evident that, in respect of that which completes a sin's species, sins are not connected with one another. For sin does not consist in passing from the many to the one, as is the case with virtues, which are connected, but rather in forsaking the one for the many.
Reply to Objection 1. James is speaking of sin, not as regards the thing to which it turns and which causes the distinction of sins, as stated above (72, 1), but as regards that from which sin turns away, in as much as man, by sinning, departs from a commandment of the law. Now all the commandments of the law are from one and the same, as he also says in the same passage, so that the same God is despised in every sin; and in this sense he says that whoever "offends in one point, is become guilty of all," for as much as, by committing one sin, he incurs the debt of punishment through his contempt of God, which is the origin of all sins.
Reply to Objection 2. As stated above (Question 71, Article 4), the opposite virtue is not banished by every act of sin; because venial sin does not destroy virtue; while mortal sin destroys infused virtue, by turning man away from God. Yet one act, even of mortal sin, does not destroy the habit of acquired virtue; though if such acts be repeated so as to engender a contrary habit, the habit of acquired virtue is destroyed, the destruction of which entails the loss of prudence, since when man acts against any virtue whatever, he acts against prudence, without which no moral virtue is possible, as stated above (58, 4; 65, 1). Consequently all the moral virtues are destroyed as to the perfect and formal being of virtue, which they have in so far as they partake of prudence, yet there remain the inclinations to virtuous acts, which inclinations, however, are not virtues. Nevertheless it does not follow that for this reason man contracts all vices of sins--first, because several vices are opposed to one virtue, so that a virtue can be destroyed by one of them, without the others being present; secondly, because sin is directly opposed to virtue, as regards the virtue's inclination to act, as stated above (Question 71, Article 1). Wherefore, as long as any virtuous inclinations remain, it cannot be said that man has the opposite vices or sins.
Reply to Objection 3. The love of God is unitive, in as much as it draws man's affections from the many to the one; so that the virtues, which flow from the love of God, are connected together. But self-love disunites man's affections among different things, in so far as man loves himself, by desiring for himself temporal goods, which are various and of many kinds: hence vices and sins, which arise from self-love, are not connected together.
Source: Summa Theologica (New Advent)