Objection 1. It would seem that the act of sin is not from God. For Augustine says (De Perfect. Justit. ii) that "the act of sin is not a thing." Now whatever is from God is a thing. Therefore the act of sin is not from God.
Objection 2. Further, man is not said to be the cause of sin, except because he is the cause of the sinful act: for "no one works, intending evil," as Dionysius states (Div. Nom. iv). Now God is not a cause of sin, as stated above (Article 1). Therefore God is not the cause of the act of sin.
Objection 3. Further, some actions are evil and sinful in their species, as was shown above (18, A2,8). Now whatever is the cause of a thing, causes whatever belongs to it in respect of its species. If therefore God caused the act of sin, He would be the cause of sin, which is false, as was proved above (Article 1). Therefore God is not the cause of the act of sin.
On the contrary, The act of sin is a movement of the free-will. Now "the will of God is the cause of every movement," as Augustine declares (De Trin. iii, 4,9). Therefore God's will is the cause of the act of sin.
I answer that, The act of sin is both a being and an act; and in both respects it is from God. Because every being, whatever the mode of its being, must be derived from the First Being, as Dionysius declares (Div. Nom. v). Again every action is caused by something existing in act, since nothing produces an action save in so far as it is in act; and every being in act is reduced to the First Act, viz. God, as to its cause, Who is act by His Essence. Therefore God is the cause of every action, in so far as it is an action. But sin denotes a being and an action with a defect: and this defect is from the created cause, viz. the free-will, as falling away from the order of the First Agent, viz. God. Consequently this defect is not reduced to God as its cause, but to the free-will: even as the defect of limping is reduced to a crooked leg as its cause, but not to the motive power, which nevertheless causes whatever there is of movement in the limping. Accordingly God is the cause of the act of sin: and yet He is not the cause of sin, because He does not cause the act to have a defect.
Reply to Objection 1. In this passage Augustine calls by the name of "thing," that which is a thing simply, viz. substance; for in this sense the act of sin is not a thing.
Reply to Objection 2. Not only the act, but also the defect, is reduced to man as its cause, which defect consists in man not being subject to Whom he ought to be, although he does not intend this principally. Wherefore man is the cause of the sin: while God is the cause of the act, in such a way, that nowise is He the cause of the defect accompanying the act, so that He is not the cause of the sin.
Reply to Objection 3. As stated above (Question 72, Article 1), acts and habits do not take their species from the privation itself, wherein consists the nature of evil, but from some object, to which that privation is united: and so this defect which consists in not being from God, belongs to the species of the act consequently, and not as a specific difference.
Source: Summa Theologica (New Advent)