Objection 1. It would seem that the justification of the ungodly does not take place in an instant, but successively, since, as already stated (3), for the justification of the ungodly, there is required a movement of free-will. Now the act of the free-will is choice, which requires the deliberation of counsel, as stated above (Question 13, Article 1). Hence, since deliberation implies a certain reasoning process, and this implies succession, the justification of the ungodly would seem to be successive.
Objection 2. Further, the free-will's movement is not without actual consideration. But it is impossible to understand many things actually and at once, as stated above (I, 85, 4). Hence, since for the justification of the ungodly there is required a movement of the free-will towards several things, viz. towards God and towards sin, it would seem impossible for the justification of the ungodly to be in an instant.
Objection 3. Further, a form that may be greater or less, e.g. blackness or whiteness, is received successively by its subject. Now grace may be greater or less, as stated above (Question 112, Article 4). Hence it is not received suddenly by its subject. Therefore, seeing that the infusion of grace is required for the justification of the ungodly, it would seem that the justification of the ungodly cannot be in an instant.
Objection 4. Further, the free-will's movement, which cooperates in justification, is meritorious; and hence it must proceed from grace, without which there is no merit, as we shall state further on (114, 2). Now a thing receives its form before operating by this form. Hence grace is first infused, and then the free-will is moved towards God and to detest sin. Hence justification is not all at once.
Objection 5. Further, if grace is infused into the soul, there must be an instant when it first dwells in the soul; so, too, if sin is forgiven there must be a last instant that man is in sin. But it cannot be the same instant, otherwise opposites would be in the same simultaneously. Hence they must be two successive instants; between which there must be time, as the Philosopher says (Phys. vi, 1). Therefore the justification of the ungodly takes place not all at once, but successively.
On the contrary, The justification of the ungodly is caused by the justifying grace of the Holy Spirit. Now the Holy Spirit comes to men's minds suddenly, according to Acts 2:2: "And suddenly there came a sound from heaven as of a mighty wind coming," upon which the gloss says that "the grace of the Holy Ghost knows no tardy efforts." Hence the justification of the ungodly is not successive, but instantaneous.
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 (112, 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 (112, 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 to Objection 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 to Objection 2. As stated above (I, 85, 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 to Objection 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.
Reply to Objection 4. The same instant the form is acquired, the thing begins to operate with the form; as fire, the instant it is generated moves upwards, and if its movement was instantaneous, it would be terminated in the same instant. Now to will and not to will--the movements of the free-will--are not successive, but instantaneous. Hence the justification of the ungodly must not be successive.
Reply to Objection 5. The succession of opposites in the same subject must be looked at differently in the things that are subject to time and in those that are above time. For in those that are in time, there is no last instant in which the previous form inheres in the subject; but there is the last time, and the first instant that the subsequent form inheres in the matter or subject; and this for the reason, that in time we are not to consider one instant, since neither do instants succeed each other immediately in time, nor points in a line, as is proved in Physic. vi, 1. But time is terminated by an instant. Hence in the whole of the previous time wherein anything is moving towards its form, it is under the opposite form; but in the last instant of this time, which is the first instant of the subsequent time, it has the form which is the term of the movement.
But in those that are above time, it is otherwise. For if there be any succession of affections or intellectual conceptions in them (as in the angels), such succession is not measured by continuous time, but by discrete time, even as the things measured are not continuous, as stated above (I, 53, 2,3). In these, therefore, there is a last instant in which the preceding is, and a first instant in which the subsequent is. Nor must there be time in between, since there is no continuity of time, which this would necessitate.
Now the human mind, which is justified, is, in itself, above time, but is subject to time accidentally, inasmuch as it understands with continuity and time, with respect to the phantasms in which it considers the intelligible species, as stated above (I, 85, A1,2). We must, therefore, decide from this about its change as regards the condition of temporal movements, i.e. we must say that there is no last instant that sin inheres, but a last time; whereas there is a first instant that grace inheres; and in all the time previous sin inhered.
Source: Summa Theologica (New Advent)