In ST IaIIae q. 113, a. 6, Thomas answers that there are four things necessary for justification:
1. Infusion of graceThis follows because there must be a mover, something moved, and the end for which there is a motion. The mover is God who operates through the infusion of grace. That which is moved is the free-will in the double movement of the free-will disposing itself toward God and away from sin. The remission of sin is the goal of the movement. Thomas calls this the “natural order” of justification (a. 8, c).
2. Movement of free-will toward God
3. Movement of free-will from sin
4. Remission of sins
A difficulty arises in how one understands that which is moved, i.e. the free-will. Thomas grants that there are two ways of looking at the problem. First, one might look at it from the perspective of the thing moved. From point of view of that which is moved, the free-will logically first moves from sin and then to God. However, from the point of view of the agent (who is God) the order is reversed, because in the agent the form is pre-existing. The example given is that the sun with respect to removing darkness holds illumination as prior to dispelling darkness.
These distinctions regard only the natural order and do not indicate a temporal order of events. The succession of opposites is a philosophical problem that Thomas wrestles with in the replies to a. 7. It relates to the question of “instants” following “instants”. This relates to Zeno’s paradox of an infinitely divisible space or time. Thomas, following Aristotle, simply states that previous time can be terminated by an instant. The last instant of the previous time because the first instant of the subsequent time. In other words, the moment the ungodly man is justified, he is instantly infused with grace, inclined to God, away from sin, and all sins are remitted.
Thus, the four elements of justification occur instantly and simultaneously in time although logically they follow the natural order delineated by Thomas.