Objection 1: It would seem that all sins are due to the temptation of the devil. For Dionysius says (Div. Nom. iv) that "the multitude of demons is the cause of all evils, both to themselves and to others." And Damascene says (De Fide Orth. ii, 4) that "all malice and all uncleanness have been devised by the devil."
Obj. 2: Further, of every sinner can be said what the Lord said of the Jews (John 8:44): "You are of your father the devil." But this was in as far as they sinned through the devil's instigation. Therefore every sin is due to the devil's instigation.
Obj. 3: Further, as angels are deputed to guard men, so demons are deputed to assail men. But every good thing we do is due to the suggestion of the good angels: because the Divine gifts are borne to us by the angels. Therefore all the evil we do, is due to the instigation of the devil.
On the contrary, It is written (De Eccl. Dogmat. xlix): "Not all our evil thoughts are stirred up by the devil, but sometimes they arise from the movement of our free-will."
I answer that, One thing can be the cause of another in two ways; directly and indirectly. Indirectly as when an agent is the cause of a disposition to a certain effect, it is said to be the occasional and indirect cause of that effect: for instance, we might say that he who dries the wood is the cause of the wood burning. In this way we must admit that the devil is the cause of all our sins; because he it was who instigated the first man to sin, from whose sin there resulted a proneness to sin in the whole human race: and in this sense we must take the words of Damascene and Dionysius.
But a thing is said to be the direct cause of something, when its action tends directly thereunto. And in this way the devil is not the cause of every sin: for all sins are not committed at the devil's instigation, but some are due to the free-will and the corruption of the flesh. For, as Origen says (Peri Archon iii), even if there were no devil, men would have the desire for food and love and such like pleasures; with regard to which many disorders may arise unless those desires are curbed by reason, especially if we presuppose the corruption of our natures. Now it is in the power of the free-will to curb this appetite and keep it in order. Consequently there is no need for all sins to be due to the instigation of the devil. But those sins which are due thereto man perpetrates "through being deceived by the same blandishments as were our first parents," as Isidore says (De Summo Bono ii).
Thus the answer to the first objection is clear.
Reply Obj. 2: When man commits sin without being thereto instigated by the devil, he nevertheless becomes a child of the devil thereby, in so far as he imitates him who was the first to sin.
Reply Obj. 3: Man can of his own accord fall into sin: but he cannot advance in merit without the Divine assistance, which is borne to man by the ministry of the angels. For this reason the angels take part in all our good works: whereas all our sins are not due to the demons' instigation. Nevertheless there is no kind of sin which is not sometimes due to the demons' suggestion.