Objection 1. It seems that grace and virtues are not bestowed on man by Baptism. Because, as stated above (62, 1, ad 1), the sacraments of the New Law "effect what they signify." But the baptismal cleansing signifies the cleansing of the soul from guilt, and not the fashioning of the soul with grace and virtues. Therefore it seems that grace and virtues are not bestowed on man by Baptism.
Objection 2. Further, one does not need to receive what one has already acquired. But some approach Baptism who have already grace and virtues: thus we read (Acts 10:1-2): "There was a certain man in Cesarea, named Cornelius, a centurion of that which is called Italian band, a religious man and fearing God"; who, nevertheless, was afterwards baptized by Peter. Therefore grace and virtues are not bestowed by Baptism.
Objection 3. Further, virtue is a habit: which is defined as a "quality not easily removed, by which one may act easily and pleasurably." But after Baptism man retains proneness to evil which removes virtue; and experiences difficulty in doing good, in which the act of virtue consists. Therefore man does not acquire grace and virtue in Baptism.
On the contrary, The Apostle says (Titus 3:5-6): "He saved us by the laver of regeneration," i.e. by Baptism, "and renovation of the Holy Ghost, Whom He hath poured forth upon us abundantly," i.e. "unto the remission of sins and the fulness of virtues," as a gloss expounds. Therefore the grace of the Holy Ghost and the fulness of virtues are given in Baptism.
I answer that, As Augustine says in the book on Infant Baptism (De Pecc. Merit. et Remiss. i) "the effect of Baptism is that the baptized are incorporated in Christ as His members." Now the fulness of grace and virtues flows from Christ the Head to all His members, according to John 1:16: "Of His fulness we all have received." Hence it is clear that man receives grace and virtues in Baptism.
Reply to Objection 1. As the baptismal water by its cleansing signifies the washing away of guilt, and by its refreshment the remission of punishment, so by its natural clearness it signifies the splendor of grace and virtues.
Reply to Objection 2. As stated above (1, ad 2; 68, 2) man receives the forgiveness of sins before Baptism in so far as he has Baptism of desire, explicitly or implicitly; and yet when he actually receives Baptism, he receives a fuller remission, as to the remission of the entire punishment. So also before Baptism Cornelius and others like him receive grace and virtues through their faith in Christ and their desire for Baptism, implicit or explicit: but afterwards when baptized, they receive a yet greater fulness of grace and virtues. Hence in Psalm 22:2, "He hath brought me up on the water of refreshment," a gloss says: "He has brought us up by an increase of virtue and good deeds in Baptism."
Reply to Objection 3. Difficulty in doing good and proneness to evil are in the baptized, not through their lacking the habits of the virtues, but through concupiscence which is not taken away in Baptism. But just as concupiscence is diminished by Baptism, so as not to enslave us, so also are both the aforesaid defects diminished, so that man be not overcome by them.
Source: Summa Theologica (New Advent)