Objection 1. It seems that Baptism does not produce its effect, when the insincerity ceases. For a dead work, which is void of charity, can never come to life. But he who approaches Baptism insincerely, receives the sacrament without charity. Therefore it can never come to life so as to bestow grace.
Objection 2. Further, insincerity seems to be stronger than Baptism, because it hinders its effect. But the stronger is not removed by the weaker. Therefore the sin of insincerity cannot be taken away by Baptism which has been hindered by insincerity. And thus Baptism will not receive its full effect, which is the remission of all sins.
Objection 3. Further, it may happen that a man approach Baptism insincerely, and afterwards commit a number of sins. And yet these sins will not be taken away by Baptism; because Baptism washes away past, not future, sins. Such a Baptism, therefore, will never have its effect, which is the remission of all sins.
On the contrary, Augustine says (De Bapt. cont. Donat. i): "Then does Baptism begin to have its salutary effect, when truthful confession takes the place of that insincerity which hindered sins from being washed away, so long as the heart persisted in malice and sacrilege."
I answer that, As stated above (Question 66, Article 9), Baptism is a spiritual regeneration. Now when a thing is generated, it receives together with the form, the form's effect, unless there be an obstacle; and when this is removed, the form of the thing generated produces its effect: thus at the same time as a weighty body is generated, it has a downward movement, unless something prevent this; and when the obstacle is removed, it begins forthwith to move downwards. In like manner when a man is baptized, he receives the character, which is like a form; and he receives in consequence its proper effect, which is grace whereby all his sins are remitted. But this effect is sometimes hindered by insincerity. Wherefore, when this obstacle is removed by Penance, Baptism forthwith produces its effect.
Reply to Objection 1. The sacrament of Baptism is the work of God, not of man. Consequently, it is not dead in the man, who being insincere, is baptized without charity.
Reply to Objection 2. Insincerity is not removed by Baptism but by Penance: and when it is removed, Baptism takes away all guilt, and all debt of punishment due to sins, whether committed before Baptism, or even co-existent with Baptism. Hence Augustine says (De Bapt. cont. Donat. i): "Yesterday is blotted out, and whatever remains over and above, even the very last hour and moment preceding Baptism, the very moment of Baptism. But from that moment forward he is bound by his obligations." And so both Baptism and Penance concur in producing the effect of Baptism, but Baptism as the direct efficient cause, Penance as the indirect cause, i.e. as removing the obstacle.
Reply to Objection 3. The effect of Baptism is to take away not future, but present and past sins. And consequently, when the insincerity passes away, subsequent sins are indeed remitted, but by Penance, not by Baptism. Wherefore they are not remitted, like the sins which preceded Baptism, as to the whole debt of punishment.
Source: Summa Theologica (New Advent)