Objection 1. It would seem that the devil is directly the cause of man's sinning. For sin consists directly in an act of the appetite. Now Augustine says (De Trin. iv, 12) that "the devil inspires his friends with evil desires"; and Bede, commenting on Acts 5:3, says that the devil "draws the mind to evil desires"; and Isidore says (De Summo Bono ii, 41; iii, 5) that the devil "fills men's hearts with secret lusts." Therefore the devil is directly the cause of sin.
Objection 2. Further, Jerome says (Contra Jovin. ii, 2) that "as God is the perfecter of good, so is the devil the perfecter of evil." But God is directly the cause of our good. Therefore the devil is directly the cause of our sins.
Objection 3. Further, the Philosopher says in a chapter of the Eudemein Ethics (vii, 18): "There must needs be some extrinsic principle of human counsel." Now human counsel is not only about good things but also about evil things. Therefore, as God moves man to take good counsel, and so is the cause of good, so the devil moves him to take evil counsel, and consequently is directly the cause of sin.
On the contrary, Augustine proves (De Lib. Arb. i, 11) that "nothing else than his own will makes man's mind the slave of his desire." Now man does not become a slave to his desires, except through sin. Therefore the cause of sin cannot be the devil, but man's own will alone.
I answer that, Sin is an action: so that a thing can be directly the cause of sin, in the same way as anyone is directly the cause of an action; and this can only happen by moving that action's proper principle to act. Now the proper principle of a sinful action is the will, since every sin is voluntary. Consequently nothing can be directly the cause of sin, except that which can move the will to act.
Now the will, as stated above (9, A3,4,6), can be moved by two things: first by its object, inasmuch as the apprehended appetible is said to move the appetite: secondly by that agent which moves the will inwardly to will, and this is no other than the will itself, or God, as was shown above (9, A3,4,6). Now God cannot be the cause of sin, as stated above (Question 79, Article 1). Therefore it follows that in this respect, a man's will alone is directly the cause of his sin.
As regards the object, a thing may be understood as moving the will in three ways. First, the object itself which is proposed to the will: thus we say that food arouses man's desire to eat. Secondly, he that proposes or offers this object. Thirdly, he that persuades the will that the object proposed has an aspect of good, because he also, in a fashion, offers the will its proper object, which is a real or apparent good of reason. Accordingly, in the first way the sensible things, which approach from without, move a man's will to sin. In the second and third ways, either the devil or a man may incite to sin, either by offering an object of appetite to the senses, or by persuading the reason. But in none of these three ways can anything be the direct cause of sin, because the will is not, of necessity, moved by any object except the last end, as stated above (10, A1,2). Consequently neither the thing offered from without, nor he that proposes it, nor he that persuades, is the sufficient cause of sin. Therefore it follows that the devil is a cause of sin, neither directly nor sufficiently, but only by persuasion, or by proposing the object of appetite.
Reply to Objection 1. All these, and other like authorities, if we meet with them, are to be understood as denoting that the devil induces man to affection for a sin, either by suggesting to him, or by offering him objects of appetite.
Reply to Objection 2. This comparison is true in so far as the devil is somewhat the cause of our sins, even as God is in a certain way the cause of our good actions, but does not extend to the mode of causation: for God causes good things in us by moving the will inwardly, whereas the devil cannot move us in this way.
Reply to Objection 3. God is the universal principle of all inward movements of man; but that the human will be determined to an evil counsel, is directly due to the human will, and to the devil as persuading or offering the object of appetite.
Source: Summa Theologica (New Advent)