Objection 1. It would seem that the will is not moved by God alone as exterior principle. For it is natural that the inferior be moved by its superior: thus the lower bodies are moved by the heavenly bodies. But there is something which is higher than the will of man and below God, namely, the angel. Therefore man's will can be moved by an angel also, as exterior principle.
Objection 2. Further, the act of the will follows the act of the intellect. But man's intellect is reduced to act, not by God alone, but also by the angel who enlightens it, as Dionysius says (Coel. Hier. iv). For the same reason, therefore, the will also is moved by an angel.
Objection 3. Further, God is not the cause of other than good things, according to Genesis 1:31: "God saw all the things that He had made, and they were very good." If, therefore man's will were moved by God alone, it would never be moved to evil: and yet it is the will whereby "we sin and whereby we do right," as Augustine says (Retract. i, 9).
On the contrary, It is written (Philippians 2:13): "It is God Who worketh in us" [Vulg.'you'] "both to will and to accomplish."
I answer that, The movement of the will is from within, as also is the movement of nature. Now although it is possible for something to move a natural thing, without being the cause of the thing moved, yet that alone, which is in some way the cause of a thing's nature, can cause a natural movement in that thing. For a stone is moved upwards by a man, who is not the cause of the stone's nature, but this movement is not natural to the stone; but the natural movement of the stone is caused by no other than the cause of its nature. Wherefore it is said in Phys. vii, 4, that the generator moves locally heavy and light things. Accordingly man endowed with a will is sometimes moved by something that is not his cause; but that his voluntary movement be from an exterior principle that is not the cause of his will, is impossible.
Now the cause of the will can be none other than God. And this is evident for two reasons. First, because the will is a power of the rational soul, which is caused by God alone, by creation, as was stated in the I, 90, 2. Secondly, it is evident from the fact that the will is ordained to the universal good. Wherefore nothing else can be the cause of the will, except God Himself, Who is the universal good: while every other good is good by participation, and is some particular good, and a particular cause does not give a universal inclination. Hence neither can primary matter, which is potentiality to all forms, be created by some particular agent.
Reply to Objection 1. An angel is not above man in such a way as to be the cause of his will, as the heavenly bodies are the causes of natural forms, from which result the natural movements of natural bodies.
Reply to Objection 2. Man's intellect is moved by an angel, on the part of the object, which by the power of the angelic light is proposed to man's knowledge. And in this way the will also can be moved by a creature from without, as stated above (Article 4).
Reply to Objection 3. God moves man's will, as the Universal Mover, to the universal object of the will, which is good. And without this universal motion, man cannot will anything. But man determines himself by his reason to will this or that, which is true or apparent good. Nevertheless, sometimes God moves some specially to the willing of something determinate, which is good; as in the case of those whom He moves by grace, as we shall state later on (109, 2).
Source: Summa Theologica (New Advent)