Objection 1. It would seem that the will is moved of necessity by God. For every agent that cannot be resisted moves of necessity. But God cannot be resisted, because His power is infinite; wherefore it is written (Romans 9:19): "Who resisteth His will?" Therefore God moves the will of necessity.
Objection 2. Further, the will is moved of necessity to whatever it wills naturally, as stated above (02, ad 3). But "whatever God does in a thing is natural to it," as Augustine says (Contra Faust. xxvi, 3). Therefore the will wills of necessity everything to which God moves it.
Objection 3. Further, a thing is possible, if nothing impossible follows from its being supposed. But something impossible follows from the supposition that the will does not will that to which God moves it: because in that case God's operation would be ineffectual. Therefore it is not possible for the will not to will that to which God moves it. Therefore it wills it of necessity.
On the contrary, It is written (Sirach 15:14): "God made man from the beginning, and left him in the hand of his own counsel." Therefore He does not of necessity move man's will.
I answer that, As Dionysius says (Div. Nom. iv) "it belongs to Divine providence, not to destroy but to preserve the nature of things." Wherefore it moves all things in accordance with their conditions; so that from necessary causes through the Divine motion, effects follow of necessity; but from contingent causes, effects follow contingently. Since, therefore, the will is an active principle, not determinate to one thing, but having an indifferent relation to many things, God so moves it, that He does not determine it of necessity to one thing, but its movement remains contingent and not necessary, except in those things to which it is moved naturally.
Reply to Objection 1. The Divine will extends not only to the doing of something by the thing which He moves, but also to its being done in a way which is fitting to the nature of that thing. And therefore it would be more repugnant to the Divine motion, for the will to be moved of necessity, which is not fitting to its nature; than for it to be moved freely, which is becoming to its nature.
Reply to Objection 2. That is natural to a thing, which God so works in it that it may be natural to it: for thus is something becoming to a thing, according as God wishes it to be becoming. Now He does not wish that whatever He works in things should be natural to them, for instance, that the dead should rise again. But this He does wish to be natural to each thing--that it be subject to the Divine power.
Reply to Objection 3. If God moves the will to anything, it is incompatible with this supposition, that the will be not moved thereto. But it is not impossible simply. Consequently it does not follow that the will is moved by God necessarily.
Source: Summa Theologica (New Advent)