Objection 1. It would seem that grace is necessarily given to whoever prepares himself for grace, or to whoever does what he can, because, on Romans 5:1, "Being justified... by faith, let us have peace," etc. the gloss says: "God welcomes whoever flies to Him, otherwise there would be injustice with Him." But it is impossible for injustice to be with God. Therefore it is impossible for God not to welcome whoever flies to Him. Hence he receives grace of necessity.
Objection 2. Further, Anselm says (De Casu Diaboli. iii) that the reason why God does not bestow grace on the devil, is that he did not wish, nor was he prepared, to receive it. But if the cause be removed, the effect must needs be removed also. Therefore, if anyone is willing to receive grace it is bestowed on them of necessity.
Objection 3. Further, good is diffusive of itself, as appears from Dionysius (Div. Nom. iv). Now the good of grace is better than the good of nature. Hence, since natural forms necessarily come to disposed matter, much more does it seem that grace is necessarily bestowed on whoever prepares himself for grace.
On the contrary, Man is compared to God as clay to the potter, according to Jeremiah 18:6: "As clay is in the hand of the potter, so are you in My hand." But however much the clay is prepared, it does not necessarily receive its shape from the potter. Hence, however much a man prepares himself, he does not necessarily receive grace from God.
I answer that, As stated above (Article 2), man's preparation for grace is from God, as Mover, and from the free-will, as moved. Hence the preparation may be looked at in two ways: first, as it is from free-will, and thus there is no necessity that it should obtain grace, since the gift of grace exceeds every preparation of human power. But it may be considered, secondly, as it is from God the Mover, and thus it has a necessity--not indeed of coercion, but of infallibility--as regards what it is ordained to by God, since God's intention cannot fail, according to the saying of Augustine in his book on the Predestination of the Saints (De Dono Persev. xiv) that "by God's good gifts whoever is liberated, is most certainly liberated." Hence if God intends, while moving, that the one whose heart He moves should attain to grace, he will infallibly attain to it, according to John 6:45: "Every one that hath heard of the Father, and hath learned, cometh to Me."
Reply to Objection 1. This gloss is speaking of such as fly to God by a meritorious act of their free-will, already "informed" with grace; for if they did not receive grace, it would be against the justice which He Himself established. Or if it refers to the movement of free-will before grace, it is speaking in the sense that man's flight to God is by a Divine motion, which ought not, in justice, to fail.
Reply to Objection 2. The first cause of the defect of grace is on our part; but the first cause of the bestowal of grace is on God's according to Hosea 13:9: "Destruction is thy own, O Israel; thy help is only in Me."
Reply to Objection 3. Even in natural things, the form does not necessarily ensue the disposition of the matter, except by the power of the agent that causes the disposition.
Source: Summa Theologica (New Advent)