Objection 1. It would seem that all the sins of men are due to the devil's suggestion. For Dionysius says (Div. Nom. iv) that the "crowd of demons are the cause of all evils, both to themselves and to others."
Objection 2. Further, whoever sins mortally, becomes the slave of the devil, according to John 8:34: "Whosoever committeth sin is the slave [Douay: 'servant'] of sin." Now "by whom a man is overcome, of the same also he is the slave" (2 Peter 2:19). Therefore whoever commits a sin, has been overcome by the devil.
Objection 3. Further, Gregory says (Moral. iv, 10) the sin of the devil is irreparable, because he sinned at no other's suggestion. Therefore, if any men were to sin of their own free-will and without suggestion from any other, their sin would be irremediable: which is clearly false. Therefore all the sins of men are due to the devil's suggestion.
On the contrary, It is written (De Eccl. Dogm. lxxxii): "Not all our evil thoughts are incited by the devil; sometimes they are due to a movement of the free-will."
I answer that, the devil is the occasional and indirect cause of all our sins, in so far as he induced the first man to sin, by reason of whose sin human nature is so infected, that we are all prone to sin: even as the burning of wood might be imputed to the man who dried the wood so as to make it easily inflammable. He is not, however, the direct cause of all the sins of men, as though each were the result of his suggestion. Origen proves this (Peri Archon iii, 2) from the fact that even if the devil were no more, men would still have the desire for food, sexual pleasures and the like; which desire might be inordinate, unless it were subordinate to reason, a matter that is subject to the free-will.
Reply to Objection 1. The crowd of demons are the cause of all our evils, as regards their original cause, as stated.
Reply to Objection 2. A man becomes another's slave not only by being overcome by him, but also by subjecting himself to him spontaneously: it is thus that one who sins of his own accord, becomes the slave of the devil.
Reply to Objection 3. The devil's sin was irremediable, not only because he sinned without another's suggestion; but also because he was not already prone to sin, on account of any previous sin; which can be said of no sin of man.
Source: Summa Theologica (New Advent)