Objection 1. It seems that insincerity does not hinder the effect of Baptism. For the Apostle says (Galatians 3:27): "As many of you as have been baptized in Christ Jesus, have put on Christ." But all that receive the Baptism of Christ, are baptized in Christ. Therefore they all put on Christ: and this is to receive the effect of Baptism. Consequently insincerity does not hinder the effect of Baptism.
Objection 2. Further, the Divine power which can change man's will to that which is better, works in Baptism. But the effect of the efficient cause cannot be hindered by that which can be removed by that cause. Therefore insincerity cannot hinder the effect of Baptism.
Objection 3. Further, the effect of Baptism is grace, to which sin is in opposition. But many other sins are more grievous than insincerity, which are not said to hinder the effect of Baptism. Therefore neither does insincerity.
On the contrary, It is written (Wisdom 1:5): "The Holy Spirit of discipline will flee from the deceitful." But the effect of Baptism is from the Holy Ghost. Therefore insincerity hinders the effect of Baptism.
I answer that, As Damascene says (De Fide Orth. ii), "God does not compel man to be righteous." Consequently in order that a man be justified by Baptism, his will must needs embrace both Baptism and the baptismal effect. Now, a man is said to be insincere by reason of his will being in contradiction with either Baptism or its effect. For, according to Augustine (De Bapt. cont. Donat. vii), a man is said to be insincere, in four ways: first, because he does not believe, whereas Baptism is the sacrament of Faith; secondly, through scorning the sacrament itself; thirdly, through observing a rite which differs from that prescribed by the Church in conferring the sacrament; fourthly, through approaching the sacrament without devotion. Wherefore it is manifest that insincerity hinders the effect of Baptism.
Reply to Objection 1. "To be baptized in Christ," may be taken in two ways. First, "in Christ," i.e. "in conformity with Christ." And thus whoever is baptized in Christ so as to be conformed to Him by Faith and Charity, puts on Christ by grace. Secondly, a man is said to be baptized in Christ, in so far as he receives Christ's sacrament. And thus all put on Christ, through being configured to Him by the character, but not through being conformed to Him by grace.
Reply to Objection 2. When God changes man's will from evil to good, man does not approach with insincerity. But God does not always do this. Nor is this the purpose of the sacrament, that an insincere man be made sincere; but that he who comes in sincerity, be justified.
Reply to Objection 3. A man is said to be insincere who makes a show of willing what he wills not. Now whoever approaches Baptism, by that very fact makes a show of having right faith in Christ, of veneration for this sacrament, and of wishing to conform to the Church, and to renounce sin. Consequently, to whatever sin a man wishes to cleave, if he approach Baptism, he approaches insincerely, which is the same as to approach without devotion. But this must be understood of mortal sin, which is in opposition to grace: but not of venial sin. Consequently, here insincerity includes, in a way, every sin.
Source: Summa Theologica (New Advent)