It does not erase the mark but "prevents Baptism from bearing the fruits of salvation." From the Catechism of the Catholic Church:
1855 Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.What is at issue is the very nature of salvation. For the Protestant it is the forensic application of God's justice upon the sinner. For the Catholic it is the cleansing of not just the penalty of sin but of sin itself; the imparting of God's holiness, not just its declaration. You will excuse me if I give a longer passage from the Catechism:
Venial sin allows charity to subsist, even though it offends and wounds it.1856 Mortal sin, by attacking the vital principle within us - that is, charity - necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation:
When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object . . . whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery.... But when the sinner's will is set upon something that of its nature involves a disorder, but is not opposed to the love of God and neighbor, such as thoughtless chatter or immoderate laughter and the like, such sins are venial.
I. Justification1987 The grace of the Holy Spirit has the power to justify us, that is, to cleanse us from our sins and to communicate to us "the righteousness of God through faith in Jesus Christ" and through Baptism: (Rom 3:22; cf. 6:3-4)
But if we have died with Christ, we believe that we shall also live with him. For we know that Christ being raised from the dead will never die again; death no longer has dominion over him. the death he died he died to sin, once for all, but the life he lives he lives to God. So you also must consider yourselves as dead to sin and alive to God in Christ Jesus. (Rom 6:8-11)1988 Through the power of the Holy Spirit we take part in Christ's Passion by dying to sin, and in his Resurrection by being born to a new life; we are members of his Body which is the Church, branches grafted onto the vine which is himself: (Cf. 1 Cor 12; Jn 15:1 4)(God) gave himself to us through his Spirit. By the participation of the Spirit, we become communicants in the divine nature.... For this reason, those in whom the Spirit dwells are divinized. (St. Athanasius, Ep. Serap. 1, 24: PG 26, 585 and 588)1989 The first work of the grace of the Holy Spirit is conversion, effecting justification in accordance with Jesus' proclamation at the beginning of the Gospel: "Repent, for the kingdom of heaven is at hand." (Mt 4:17) Moved by grace, man turns toward God and away from sin, thus accepting forgiveness and righteousness from on high. "Justification is not only the remission of sins, but also the sanctification and renewal of the interior man. (Council of Trent (1547): DS 1528)1990 Justification detaches man from sin which contradicts the love of God, and purifies his heart of sin. Justification follows upon God's merciful initiative of offering forgiveness. It reconciles man with God. It frees from the enslavement to sin, and it heals.
1991 Justification is at the same time the acceptance of God's righteousness through faith in Jesus Christ. Righteousness (or "justice") here means the rectitude of divine love. With justification, faith, hope, and charity are poured into our hearts, and obedience to the divine will is granted us.
1992 Justification has been merited for us by the Passion of Christ who offered himself on the cross as a living victim, holy and pleasing to God, and whose blood has become the instrument of atonement for the sins of all men. Justification is conferred in Baptism, the sacrament of faith. It conforms us to the righteousness of God, who makes us inwardly just by the power of his mercy. Its purpose is the glory of God and of Christ, and the gift of eternal life: (Cf. Council of Trent (1547): DS 1529)
But now the righteousness of God has been manifested apart from law, although the law and the prophets bear witness to it, the righteousness of God through faith in Jesus Christ for all who believe. For there is no distinction: since all have sinned and fall short of the glory of God, they are justified by his grace as a gift, through the redemption which is in Christ Jesus, whom God put forward as an expiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins; it was to prove at the present time that he himself is righteous and that he justifies him who has faith in Jesus. (Rom 3:21-26)1993 Justification establishes cooperation between God's grace and man's freedom. On man's part it is expressed by the assent of faith to the Word of God, which invites him to conversion, and in the cooperation of charity with the prompting of the Holy Spirit who precedes and preserves his assent:When God touches man's heart through the illumination of the Holy Spirit, man himself is not inactive while receiving that inspiration, since he could reject it; and yet, without God's grace, he cannot by his own free will move himself toward justice in God's sight. (Council of Trent (1547): DS 1525)1994 Justification is the most excellent work of God's love made manifest in Christ Jesus and granted by the Holy Spirit. It is the opinion of St. Augustine that "the justification of the wicked is a greater work than the creation of heaven and earth," because "heaven and earth will pass away but the salvation and justification of the elect . . . will not pass away." (St. Augustine, In Jo. ev. 72, 3: PL 35, 1823) He holds also that the justification of sinners surpasses the creation of the angels in justice, in that it bears witness to a greater mercy.1995 The Holy Spirit is the master of the interior life. By giving birth to the "inner man," (Cf. Rom 7:22; Eph 3:16) justification entails the sanctification of his whole being:
Just as you once yielded your members to impurity and to greater and greater iniquity, so now yield your members to righteousness for sanctification.... But now that you have been set free from sin and have become slaves of God, the return you get is sanctification and its end, eternal life. (Rom 6:19, 22)
Sure seems then that baptism doesn't "save" someone if a thought can erase it all.
Remember...Jesus equated lust after a woman as adultery. Just the thought of it...not the actual act itself.
.1856 Mortal sin, by attacking the vital principle within us - that is, charity - necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation:
So you're starting over again where you were to begin with...a sinner separated from God.
Then why not baptism again if that cleanses you?
What is at issue is the very nature of salvation. For the Protestant it is the forensic application of God's justice upon the sinner. For the Catholic it is the cleansing of not just the penalty of sin but of sin itself; the imparting of God's holiness, not just its declaration.
Not sure who these Protestants are you keep referring to...but Christians believe we are forgiven of sins and conformed to His image.
29For those whom He foreknew, He also predestined to become conformed to the image of His Son, so that He would be the firstborn among many brethren; Romans 8:29
Paul writes in Ephesians 1:13-14 that we are "sealed with the Holy Spirit of promise". I can find no where the believer in Christ is ever unsealed.
God makes salvation simple.
John 1:10-13 He was in the world, and the world was made through him, yet the world did not know him. He came to his own, and his own people did not receive him. But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God.
Romans 10:9-13 because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved. For the Scripture says, “Everyone who believes in him will not be put to shame.” For there is no distinction between Jew and Greek; for the same Lord is Lord of all, bestowing his riches on all who call on him. For “everyone who calls on the name of the Lord will be saved.”