Objection 1. It would seem that sin has no cause. For sin has the nature of evil, as stated above (Question 71, Article 6). But evil has no cause, as Dionysius says (Div. Nom. iv). Therefore sin has no cause.
Objection 2. Further, a cause is that from which something follows of necessity. Now that which is of necessity, seems to be no sin, for every sin is voluntary. Therefore sin has no cause.
Objection 3. Further, if sin has a cause, this cause is either good or evil. It is not a good, because good produces nothing but good, for "a good tree cannot bring forth evil fruit" (Matthew 7:18). Likewise neither can evil be the cause of sin, because the evil of punishment is a sequel to sin, and the evil of guilt is the same as sin. Therefore sin has no cause.
On the contrary, Whatever is done has a cause, for, according to Job 5:6, "nothing upon earth is done without a cause." But sin is something done; since it a "word, deed, or desire contrary to the law of God." Therefore sin has a cause.
I answer that, A sin is an inordinate act. Accordingly, so far as it is an act, it can have a direct cause, even as any other act; but, so far as it is inordinate, it has a cause, in the same way as a negation or privation can have a cause. Now two causes may be assigned to a negation: in the first place, absence of the cause of affirmation; i.e. the negation of the cause itself, is the cause of the negation in itself; since the result of the removing the cause is the removal of the effect: thus the absence of the sun is the cause of darkness. In the second place, the cause of an affirmation, of which a negation is a sequel, is the accidental cause of the resulting negation: thus fire by causing heat in virtue of its principal tendency, consequently causes a privation of cold. The first of these suffices to cause a simple negation. But, since the inordinateness of sin and of every evil is not a simple negation, but the privation of that which something ought naturally to have, such an inordinateness must needs have an accidental efficient cause. For that which naturally is and ought to be in a thing, is never lacking except on account of some impeding cause. And accordingly we are wont to say that evil, which consists in a certain privation, has a deficient cause, or an accidental efficient cause. Now every accidental cause is reducible to the direct cause. Since then sin, on the part of its inordinateness, has an accidental efficient cause, and on the part of the act, a direct efficient cause, it follows that the inordinateness of sin is a result of the cause of the act. Accordingly then, the will lacking the direction of the rule of reason and of the Divine law, and intent on some mutable good, causes the act of sin directly, and the inordinateness of the act, indirectly, and beside the intention: for the lack of order in the act results from the lack of direction in the will.
Reply to Objection 1. Sin signifies not only the privation of good, which privation is its inordinateness, but also the act which is the subject of that privation, which has the nature of evil: and how this evil has a cause, has been explained.
Reply to Objection 2. If this definition is to be verified in all cases, it must be understood as applying to a cause which is sufficient and not impeded. For it happens that a thing is the sufficient cause of something else, and that the effect does not follow of necessity, on account of some supervening impediment: else it would follow that all things happen of necessity, as is proved in Metaph. vi, text. 5. Accordingly, though sin has a cause, it does not follow that this is a necessary cause, since its effect can be impeded.
Reply to Objection 3. As stated above, the will in failing to apply the rule of reason or of the Divine law, is the cause of sin. Now the fact of not applying the rule of reason or of the Divine law, has not in itself the nature of evil, whether of punishment or of guilt, before it is applied to the act. Wherefore accordingly, evil is not the cause of the first sin, but some good lacking some other good.
Source: Summa Theologica (New Advent)