Objection 1. It seems that sinners should be baptized. For it is written (Zechariah 13:1): "In that day there shall be a fountain open to the House of David, and to the inhabitants of Jerusalem: for the washing of the sinner and of the unclean woman": and this is to be understood of the fountain of Baptism. Therefore it seems that the sacrament of Baptism should be offered even to sinners.
Objection 2. Further, our Lord said (Matthew 9:12): "They that are in health need not a physician, but they that are ill." But they that are ill are sinners. Therefore since Baptism is the remedy of Christ the physician of our souls, it seems that this sacrament should be offered to sinners.
Objection 3. Further, no assistance should be withdrawn from sinners. But sinners who have been baptized derive spiritual assistance from the very character of Baptism, since it is a disposition to grace. Therefore it seems that the sacrament of Baptism should be offered to sinners.
On the contrary, Augustine says (Serm. clxix): "He Who created thee without thee, will not justify thee without thee." But since a sinner's will is ill-disposed, he does not co-operate with God. Therefore it is useless to employ Baptism as a means of justification.
I answer that, A man may be said to be a sinner in two ways. First, on account of the stain and the debt of punishment incurred in the past: and on sinners in this sense the sacrament of Baptism should be conferred, since it is instituted specially for this purpose, that by it the uncleanness of sin may be washed away, according to Ephesians 5:26: "Cleansing it by the laver of water in the word of life."
Secondly, a man may be called a sinner because he wills to sin and purposes to remain in sin: and on sinners in this sense the sacrament of Baptism should not be conferred. First, indeed, because by Baptism men are incorporated in Christ, according to Galatians 3:27: "As many of you as have been baptized in Christ, have put on Christ." Now so long as a man wills to sin, he cannot be united to Christ, according to 2 Corinthians 6:14: "What participation hath justice with injustice?" Wherefore Augustine says in his book on Penance (Serm. cccli) that "no man who has the use of free-will can begin the new life, except he repent of his former life." Secondly, because there should be nothing useless in the works of Christ and of the Church. Now that is useless which does not reach the end to which it is ordained; and, on the other hand, no one having the will to sin can, at the same time, be cleansed from sin, which is the purpose of Baptism; for this would be to combine two contradictory things. Thirdly, because there should be no falsehood in the sacramental signs. Now a sign is false if it does not correspond with the thing signified. But the very fact that a man presents himself to be cleansed by Baptism, signifies that he prepares himself for the inward cleansing: while this cannot be the case with one who purposes to remain in sin. Therefore it is manifest that on such a man the sacrament of Baptism is not to be conferred.
Reply to Objection 1. The words quoted are to be understood of those sinners whose will is set on renouncing sin.
Reply to Objection 2. The physician of souls, i.e. Christ, works in two ways. First, inwardly, by Himself: and thus He prepares man's will so that it wills good and hates evil. Secondly, He works through ministers, by the outward application of the sacraments: and in this way His work consists in perfecting what was begun outwardly. Therefore the sacrament of Baptism is not to be conferred save on those in whom there appears some sign of their interior conversion: just as neither is bodily medicine given to a sick man, unless he show some sign of life.
Reply to Objection 3. Baptism is the sacrament of faith. Now dead faith does not suffice for salvation; nor is it the foundation, but living faith alone, "that worketh by charity" (Galatians 5:6), as Augustine says (De Fide et oper.). Neither, therefore, can the sacrament of Baptism give salvation to a man whose will is set on sinning, and hence expels the form of faith. Moreover, the impression of the baptismal character cannot dispose a man for grace as long as he retains the will to sin; for "God compels no man to be virtuous," as Damascene says (De Fide Orth. ii).
Source: Summa Theologica (New Advent)