I answer that, The entire justification of the ungodly consists as to its origin in the infusion of grace.  For it is by grace that free-will is moved and sin is remitted.  Now the infusion of grace takes place in an instant and without succession.  And the reason of this is that if a form be not suddenly impressed upon its subject, it is either because that subject is not disposed, or because the agent needs time to dispose the subject.  Hence we see that immediately the matter is disposed by a preceding alteration, the substantial form accrues to the matter; thus because the atmosphere of itself is disposed to receive light, it is suddenly illuminated by a body actually luminous.  Now it was stated (Q. 112, A. 2) that God, in order to infuse grace into the soul, needs no disposition, save what He Himself has made.  And sometimes this sufficient disposition for the reception of grace He makes suddenly, sometimes gradually and successively, as stated above (Q. 112, A. 2, ad 2).  For the reason why a natural agent cannot suddenly dispose matter is that in the matter there is a resistant which has some disproportion with the power of the agent; and hence we see that the stronger the agent, the more speedily is the matter disposed.  Therefore, since the Divine power is infinite, it can suddenly dispose any matter whatsoever to its form; and much more man’s free-will, whose movement is by nature instantaneous.  Therefore the justification of the ungodly by God takes place in an instant.

Reply Obj. 1:  The movement of the free-will, which concurs in the justification of the ungodly, is a consent to detest sin, and to draw near to God; and this consent takes place suddenly.  Sometimes, indeed, it happens that deliberation precedes, yet this is not of the substance of justification, but a way of justification; as local movement is a way of illumination, and alteration to generation.

Reply Obj. 2:  As stated above (I, Q. 85, A. 5), there is nothing to prevent two things being understood at once, in so far as they are somehow one; thus we understand the subject and predicate together, inasmuch as they are united in the order of one affirmation.  And in the same manner can the free-will be moved to two things at once in so far as one is ordained to the other.  Now the free-will’s movement towards sin is ordained to the free-will’s movement towards God, since a man detests sin, as contrary to God, to Whom he wishes to cling.  Hence in the justification of the ungodly the free-will simultaneously detests sin and turns to God, even as a body approaches one point and withdraws from another simultaneously.

Reply Obj. 3:  The reason why a form is not received instantaneously in the matter is not the fact that it can inhere more or less; for thus the light would not be suddenly received in the air, which can be illumined more or less.  But the reason is to be sought on the part of the disposition of the matter or subject, as stated above.