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. [ST I-II, Q113, A6]
Justification begins with grace, continues with the movements of the free will toward God and away from sin, and the end of the movement in justification. It should be said, for the sake of the one who might look at this sequence and erroneously conclude that the movements of the will Aquinas mentions make the Gospel "works-based," that we have already seen how these movements do not and cannot occur apart from grace - not merely in their beginning, but likewise throughout. Because that grace by which we are justified is God's operating grace, which is God's work, not ours. And of course we have seen that man's salvation is from grace.
St. Thomas continues:
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. [ST, op. cit.]
Note once again that the will is moved by God, which is grace.