Objection 1. It would seem that God cannot move the created will. For whatever is moved from without, is forced. But the will cannot be forced. Therefore it is not moved from without; and therefore cannot be moved by God.
Objection 2. Further, God cannot make two contradictories to be true at the same time. But this would follow if He moved the will; for to be voluntarily moved means to be moved from within, and not by another. Therefore God cannot move the will.
Objection 3. Further, movement is attributed to the mover rather than to the one moved; wherefore homicide is not ascribed to the stone, but to the thrower. Therefore, if God moves the will, it follows that voluntary actions are not imputed to man for reward or blame. But this is false. Therefore God does not move the will.
On the contrary, It is written (Philippians 2:13): "It is God who worketh in us [Vulgate--'you'] both to will and to accomplish."
I answer that, As the intellect is moved by the object and by the Giver of the power of intelligence, as stated above (Article 3), so is the will moved by its object, which is good, and by Him who creates the power of willing. Now the will can be moved by good as its object, but by God alone sufficiently and efficaciously. For nothing can move a movable thing sufficiently unless the active power of the mover surpasses or at least equals the potentiality of the thing movable. Now the potentiality of the will extends to the universal good; for its object is the universal good; just as the object of the intellect is the universal being. But every created good is some particular good; God alone is the universal good. Whereas He alone fills the capacity of the will, and moves it sufficiently as its object. In like manner the power of willing is caused by God alone. For to will is nothing but to be inclined towards the object of the will, which is universal good. But to incline towards the universal good belongs to the First Mover, to Whom the ultimate end is proportionate; just as in human affairs to him that presides over the community belongs the directing of his subjects to the common weal. Wherefore in both ways it belongs to God to move the will; but especially in the second way by an interior inclination of the will.
Reply to Objection 1. A thing moved by another is forced if moved against its natural inclination; but if it is moved by another giving to it the proper natural inclination, it is not forced; as when a heavy body is made to move downwards by that which produced it, then it is not forced. In like manner God, while moving the will, does not force it, because He gives the will its own natural inclination.
Reply to Objection 2. To be moved voluntarily, is to be moved from within, that is, by an interior principle: yet this interior principle may be caused by an exterior principle; and so to be moved from within is not repugnant to being moved by another.
Reply to Objection 3. If the will were so moved by another as in no way to be moved from within itself, the act of the will would not be imputed for reward or blame. But since its being moved by another does not prevent its being moved from within itself, as we have stated (ad 2), it does not thereby forfeit the motive for merit or demerit.
Source: Summa Theologica (New Advent)