Objection 1. It seems that certain acts of the virtues are unfittingly set down as effects of Baptism, to wit--"incorporation in Christ, enlightenment, and fruitfulness." For Baptism is not given to an adult, except he believe; according to Mark 16:16: "He that believeth and is baptized, shall be saved." But it is by faith that man is incorporated in Christ, according to Ephesians 3:17: "That Christ may dwell by faith in your hearts." Therefore no one is baptized except he be already incorporated in Christ. Therefore incorporation with Christ is not the effect of Baptism.
Objection 2. Further, enlightenment is caused by teaching, according to Ephesians 3:8-9: "To me the least of all the saints, is given this grace... to enlighten all men," etc. But teaching by the catechism precedes Baptism. Therefore it is not the effect of Baptism.
Objection 3. Further, fruitfulness pertains to active generation. But a man is regenerated spiritually by Baptism. Therefore fruitfulness is not an effect of Baptism.
On the contrary, Augustine says in the book on Infant Baptism (De Pecc. Merit. et Remiss. i) that "the effect of Baptism is that the baptized are incorporated in Christ." And Dionysius (Eccl. Hier. ii) ascribes enlightenment to Baptism. And on Psalm 22:2, "He hath brought me up on the water of refreshment," a gloss says that "the sinner's soul, sterilized by drought, is made fruitful by Baptism."
I answer that, By Baptism man is born again unto the spiritual life, which is proper to the faithful of Christ, as the Apostle says (Galatians 2:20): "And that I live now in the flesh; I live in the faith of the Son of God." Now life is only in those members that are united to the head, from which they derive sense and movement. And therefore it follows of necessity that by Baptism man is incorporated in Christ, as one of His members. Again, just as the members derive sense and movement from the material head, so from their spiritual Head, i.e. Christ, do His members derive spiritual sense consisting in the knowledge Of truth, and spiritual movement which results from the instinct of grace. Hence it is written (John 1:14-16): "We have seen Him... full of grace and truth; and of His fulness we all have received." And it follows from this that the baptized are enlightened by Christ as to the knowledge of truth, and made fruitful by Him with the fruitfulness of good works by the infusion of grace.
Reply to Objection 1. Adults who already believe in Christ are incorporated in Him mentally. But afterwards, when they are baptized, they are incorporated in Him, corporally, as it were, i.e. by the visible sacrament; without the desire of which they could not have been incorporated in Him even mentally.
Reply to Objection 2. The teacher enlightens outwardly and ministerially by catechizing: but God enlightens the baptized inwardly, by preparing their hearts for the reception of the doctrines of truth, according to John 6:45: "It is written in the prophets... They shall all be taught of God."
Reply to Objection 3. The fruitfulness which i ascribed as an effect of Baptism is that by which man brings forth good works; not that by which he begets others in Christ, as the Apostle says (1 Corinthians 4:15): "In Christ Jesus by the Gospel I have begotten you."
Source: Summa Theologica (New Advent)