Objection 1. It would seem that the remission of sins ought not to be reckoned amongst the things required for justification. For the substance of a thing is not reckoned together with those that are required for a thing; thus a man is not reckoned together with his body and soul. But the justification of the ungodly is itself the remission of sins, as stated above (Article 1). Therefore the remission of sins ought not to be reckoned among the things required for the justification of the ungodly.
Objection 2. Further, infusion of grace and remission of sins are the same; as illumination and expulsion of darkness are the same. But a thing ought not to be reckoned together with itself; for unity is opposed to multitude. Therefore the remission of sins ought not to be reckoned with the infusion of grace.
Objection 3. Further, the remission of sin follows as effect from cause, from the free-will's movement towards God and sin; since it is by faith and contrition that sin is forgiven. But an effect ought not to be reckoned with its cause; since things thus enumerated together, and, as it were, condivided, are by nature simultaneous. Hence the remission of sins ought not to be reckoned with the things required for the justification of the ungodly.
On the contrary, In reckoning what is required for a thing we ought not to pass over the end, which is the chief part of everything. Now the remission of sins is the end of the justification of the ungodly; for it is written (Isaiah 27:9): "This is all the fruit, that the sin thereof should be taken away." Hence the remission of sins ought to be reckoned amongst the things required for justification.
I answer that, There are four things which are accounted to be necessary for the justification of the ungodly, viz. the infusion of grace, the movement of the free-will towards God by faith, the movement of the free-will towards sin, and the remission of sins. The reason for this is that, as stated above (Article 1), the justification of the ungodly is a movement whereby the soul is moved by God from a state of sin to a state of justice. Now in the movement whereby one thing is moved by another, three things are required: first, the motion of the mover; secondly, the movement of the moved; thirdly, the consummation of the movement, or the attainment of the end. On the part of the Divine motion, there is the infusion of grace; on the part of the free-will which is moved, there are two movements--of departure from the term "whence," and of approach to the term "whereto"; but the consummation of the movement or the attainment of the end of the movement is implied in the remission of sins; for in this is the justification of the ungodly completed.
Reply to Objection 1. The justification of the ungodly is called the remission of sins, even as every movement has its species from its term. Nevertheless, many other things are required in order to reach the term, as stated above (Article 5).
Reply to Objection 2. The infusion of grace and the remission of sin may be considered in two ways: first, with respect to the substance of the act, and thus they are the same; for by the same act God bestows grace and remits sin. Secondly, they may be considered on the part of the objects; and thus they differ by the difference between guilt, which is taken away, and grace, which is infused; just as in natural things generation and corruption differ, although the generation of one thing is the corruption of another.
Reply to Objection 3. This enumeration is not the division of a genus into its species, in which the things enumerated must be simultaneous; but it is division of the things required for the completion of anything; and in this enumeration we may have what precedes and what follows, since some of the principles and parts of a composite thing may precede and some follow.
Source: Summa Theologica (New Advent)