Objection 1. It would seem that sin is unfittingly divided into sin against God, against one's neighbor, and against oneself. For that which is common to all sins should not be reckoned as a part in the division of sin. But it is common to all sins to be against God: for it is stated in the definition of sin that it is "against God's law," as stated above (Question 66, Article 6). Therefore sin against God should not be reckoned a part of the division of sin.
Objection 2. Further, every division should consist of things in opposition to one another. But these three kinds of sin are not opposed to one another: for whoever sins against his neighbor, sins against himself and against God. Therefore sin is not fittingly divided into these three.
Objection 3. Further, specification is not taken from things external. But God and our neighbor are external to us. Therefore sins are not distinguished specifically with regard to them: and consequently sin is unfittingly divided according to these three.
On the contrary, Isidore (De Summo Bono), in giving the division of sins, says that "man is said to sin against himself, against God, and against his neighbor."
I answer that, As stated above (71, A1,6), sin is an inordinate act. Now there should be a threefold order in man: one in relation to the rule of reason, in so far as all our actions and passions should be commensurate with the rule of reason: another order is in relation to the rule of the Divine Law, whereby man should be directed in all things: and if man were by nature a solitary animal, this twofold order would suffice. But since man is naturally a civic and social animal, as is proved in Polit. i, 2, hence a third order is necessary, whereby man is directed in relation to other men among whom he has to dwell. Of these orders the second contains the first and surpasses it. For whatever things are comprised under the order of reason, are comprised under the order of God Himself. Yet some things are comprised under the order of God, which surpass the human reason, such as matters of faith, and things due to God alone. Hence he that sins in such matters, for instance, by heresy, sacrilege, or blasphemy, is said to sin against God. In like manner, the first order includes the third and surpasses it, because in all things wherein we are directed in reference to our neighbor, we need to be directed according to the order of reason. Yet in some things we are directed according to reason, in relation to ourselves only, and not in reference to our neighbor; and when man sins in these matters, he is said to sin against himself, as is seen in the glutton, the lustful, and the prodigal. But when man sins in matters concerning his neighbor, he is said to sin against his neighbor, as appears in the thief and murderer. Now the things whereby man is directed to God, his neighbor, and himself are diverse. Wherefore this distinction of sins is in respect of their objects, according to which the species of sins are diversified: and consequently this distinction of sins is properly one of different species of sins: because the virtues also, to which sins are opposed, differ specifically in respect of these three. For it is evident from what has been said (62, A1,2,3) that by the theological virtues man is directed to God; by temperance and fortitude, to himself; and by justice to his neighbor.
Reply to Objection 1. To sin against God is common to all sins, in so far as the order to God includes every human order; but in so far as order to God surpasses the other two orders, sin against God is a special kind of sin.
Reply to Objection 2. When several things, of which one includes another, are distinct from one another, this distinction is understood to refer, not to the part contained in another, but to that in which one goes beyond another. This may be seen in the division of numbers and figures: for a triangle is distinguished from a four-sided figure not in respect of its being contained thereby, but in respect of that in which it is surpassed thereby: and the same applies to the numbers three and four.
Reply to Objection 3. Although God and our neighbor are external to the sinner himself, they are not external to the act of sin, but are related to it as to its object.
Source: Summa Theologica (New Advent)