Objection 1. It would seem that no preparation or disposition for grace is required on man's part, since, as the Apostle says (Romans 4:4), "To him that worketh, the reward is not reckoned according to grace, but according to debt." Now a man's preparation by free-will can only be through some operation. Hence it would do away with the notion of grace.
Objection 2. Further, whoever is going on sinning, is not preparing himself to have grace. But to some who are going on sinning grace is given, as is clear in the case of Paul, who received grace whilst he was "breathing our threatenings and slaughter against the disciples of the Lord" (Acts 9:1). Hence no preparation for grace is required on man's part.
Objection 3. Further, an agent of infinite power needs no disposition in matter, since it does not even require matter, as appears in creation, to which grace is compared, which is called "a new creature" (Galatians 6:15). But only God, Who has infinite power, causes grace, as stated above (Article 1). Hence no preparation is required on man's part to obtain grace.
On the contrary, It is written (Amos 4:12): "Be prepared to meet thy God, O Israel," and (1 Samuel 7:3): "Prepare your hearts unto the Lord."
I answer that, As stated above (Question 111, Article 2), grace is taken in two ways: first, as a habitual gift of God. Secondly, as a help from God, Who moves the soul to good. Now taking grace in the first sense, a certain preparation of grace is required for it, since a form can only be in disposed matter. But if we speak of grace as it signifies a help from God to move us to good, no preparation is required on man's part, that, as it were, anticipates the Divine help, but rather, every preparation in man must be by the help of God moving the soul to good. And thus even the good movement of the free-will, whereby anyone is prepared for receiving the gift of grace is an act of the free-will moved by God. And thus man is said to prepare himself, according to Proverbs 16:1: "It is the part of man to prepare the soul"; yet it is principally from God, Who moves the free-will. Hence it is said that man's will is prepared by God, and that man's steps are guided by God.
Reply to Objection 1. A certain preparation of man for grace is simultaneous with the infusion of grace; and this operation is meritorious, not indeed of grace, which is already possessed--but of glory which is not yet possessed. But there is another imperfect preparation, which sometimes precedes the gift of sanctifying grace, and yet it is from God's motion. But it does not suffice for merit, since man is not yet justified by grace, and merit can only arise from grace, as will be seen further on (114, 2).
Reply to Objection 2. Since a man cannot prepare himself for grace unless God prevent and move him to good, it is of no account whether anyone arrive at perfect preparation instantaneously, or step by step. For it is written (Sirach 11:23): "It is easy in the eyes of God on a sudden to make the poor man rich." Now it sometimes happens that God moves a man to good, but not perfect good, and this preparation precedes grace. But He sometimes moves him suddenly and perfectly to good, and man receives grace suddenly, according to John 6:45: "Every one that hath heard of the Father, and hath learned, cometh to Me." And thus it happened to Paul, since, suddenly when he was in the midst of sin, his heart was perfectly moved by God to hear, to learn, to come; and hence he received grace suddenly.
Reply to Objection 3. An agent of infinite power needs no matter or disposition of matter, brought about by the action of something else; and yet, looking to the condition of the thing caused, it must cause, in the thing caused, both the matter and the due disposition for the form. So likewise, when God infuses grace into a soul, no preparation is required which He Himself does not bring about.
Source: Summa Theologica (New Advent)