Objection 1. It would seem that the infusion of grace is not what is naturally required first for the justification of the ungodly. For we withdraw from evil before drawing near to good, according to Psalm 33:15: "Turn away from evil, and do good." Now the remission of sins regards the turning away from evil, and the infusion of grace regards the turning to good. Hence the remission of sin is naturally before the infusion of grace.
Objection 2. Further, the disposition naturally precedes the form to which it disposes. Now the free-will's movement is a disposition for the reception of grace. Therefore it naturally precedes the infusion of grace.
Objection 3. Further, sin hinders the soul from tending freely to God. Now a hindrance to movement must be removed before the movement takes place. Hence the remission of sin and the free-will's movement towards sin are naturally before the infusion of grace.
On the contrary, The cause is naturally prior to its effect. Now the infusion of grace is the cause of whatever is required for the justification of the ungodly, as stated above (Article 7). Therefore it is naturally prior to it.
I answer that, The aforesaid four things required for the justification of the ungodly are simultaneous in time, since the justification of the ungodly is not successive, as stated above (Article 7); but in the order of nature, one is prior to another; and in their natural order the first is the infusion of grace; the second, the free-will's movement towards God; the third, the free-will's movement towards sin; the fourth, the remission of sin.
The reason for this is that in every movement the motion of the mover is naturally first; the disposition of the matter, or the movement of the moved, is second; the end or term of the movement in which the motion of the mover rests, is last. Now the motion of God the Mover is the infusion of grace, as stated above (Article 6); the movement or disposition of the moved is the free-will's double movement; and the term or end of the movement is the remission of sin, as stated above (Article 6). Hence in their natural order the first in the justification of the ungodly is the infusion of grace; the second is the free-will's movement towards God; the third is the free-will's movement towards sin, for he who is being justified detests sin because it is against God, and thus the free-will's movement towards God naturally precedes the free-will's movement towards sin, since it is its cause and reason; the fourth and last is the remission of sin, to which this transmutation is ordained as to an end, as stated above (1,6).
Reply to Objection 1. The withdrawal from one term and approach to another may be looked at in two ways: first, on the part of the thing moved, and thus the withdrawal from a term naturally precedes the approach to a term, since in the subject of movement the opposite which is put away is prior to the opposite which the subject moved attains to by its movement. But on the part of the agent it is the other way about, since the agent, by the form pre-existing in it, acts for the removal of the opposite form; as the sun by its light acts for the removal of darkness, and hence on the part of the sun, illumination is prior to the removal of darkness; but on the part of the atmosphere to be illuminated, to be freed from darkness is, in the order of nature, prior to being illuminated, although both are simultaneous in time. And since the infusion of grace and the remission of sin regard God Who justifies, hence in the order of nature the infusion of grace is prior to the freeing from sin. But if we look at what is on the part of the man justified, it is the other way about, since in the order of nature the being freed from sin is prior to the obtaining of justifying grace. Or it may be said that the term "whence" of justification is sin; and the term "whereto" is justice; and that grace is the cause of the forgiveness of sin and of obtaining of justice.
Reply to Objection 2. The disposition of the subject precedes the reception of the form, in the order of nature; yet it follows the action of the agent, whereby the subject is disposed. And hence the free-will's movement precedes the reception of grace in the order of nature, and follows the infusion of grace.
Reply to Objection 3. As the Philosopher says (Phys. ii, 9), in movements of the soul the movement toward the speculative principle or the practical end is the very first, but in exterior movements the removal of the impediment precedes the attainment of the end. And as the free-will's movement is a movement of the soul, in the order of nature it moves towards God as to its end, before removing the impediment of sin.
Source: Summa Theologica (New Advent)