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." [ST III, Q68, A4]
Of course, the fact that a man is a sinner is not an automatic disqualifier for the Sacrament: its very purpose is to reconcile such a man to God!
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. [ibid.; emphasis added]
From this we see that justification is not magic that happens contrary to the will of the one receiving it. God does not compel a man to become a Christian against his will.