How do Protestants Receive forgiveness for post-Baptismal Sins?

The purely Biblical stance may not reflect what particular churches or denominations have chosen to follow, but there are several important points that need to be addressed.

Baptism

Sins are not automatically washed away or forgiven by baptism. Baptism is a rite or ceremony which symbolizes that one has chosen to follow God. It is important to be baptized, as this is a public commitment--a promise to God made before witnesses--that one has chosen to unite his or her life with Christ. Baptism is the spiritual equivalent of a marriage ceremony in which the bride and groom make promises to each other as they contract marriage. The ceremony itself is powerless to keep the newly wedded couple together--that comes only through a genuine relationship.

If one is baptized without first having a relationship with Christ, the ceremony itself will be powerless to create or sustain that relationship. Conversely, entering into the covenant relationship with Christ without the baptismal ceremony is, on a spiritual level, similar to a couple living together without getting married.

The waters of baptism can symbolize cleansing from sin, or of the burial of the old man and being resurrected into newness of life. But it is just a symbol. There is a post-baptismal symbol for washing away the "dust" of sin that has accumulated in the time since baptism, and that is the foot-washing ceremony that Jesus taught his disciples in the upper room just prior to the Communion ceremony.

Jesus saith to him, He that is washed needeth not save to wash his feet, but is clean every whit: and ye are clean, but not all. (John 13:10, KJV)

Once again, this physical foot-washing is powerless to wash away actual sin--it is just a symbol.

Confession and Forgiveness

If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. (1 John 1:9, KJV)

Forgiveness, Biblically, follows confession. Confession is made to the injured party, who is the only one who can offer forgiveness. As sin is transgression of God's law, sins must be confessed to God. No human can forgive sins against God. The Pharisees understood this, which is why they accused Jesus of blasphemy when he, speaking the Father's words, offered forgiveness.

⁵ When Jesus saw their faith, he said unto the sick of the palsy, Son, your sins be forgiven you. ⁶ But there was certain of the scribes sitting there, and reasoning in their hearts, ⁷ Why does this man thus speak blasphemies? who can forgive sins but God only? (Mark 2:5-7, KJV)

The Pharisees were correct that only God could forgive sins. They were just slow to acknowledge (they were proud and jealous for the respect of the people that Jesus was gathering) that the Son of Man was the Sent of God and acting as a spokesperson for God, legitimately offering God's pardon to sin-sick souls.

Confession to People

Confess your faults one to another, and pray one for another, that ye may be healed. The effectual fervent prayer of a righteous man availeth much. (James 5:16, KJV)

James 5:16 addresses confession of faults to one another. The Greek word used in the majority of manuscripts is different here from that used in 1 John 1:9 where sins are addressed. James speaks of offenses or trespasses, using the Greek word παράπτωμα (paraptoma). In 1 John 1:9, the word used is ἁμαρτία (hamartia)--the same word used in 1 John 3:4 which gives the Biblical definition for "sin."

Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law. (1 John 3:4, KJV)

The distinction is important. If I have offended you, I need to confess my fault to you and ask for your forgiveness. When you show a forgiving spirit toward me, the Father will also be willing to forgive you of your sins against God, as is taught in The Lord's Prayer. I also must confess my sin, my transgression of God's law, to God, who only can forgive it.

This is our duty.

¹⁴ For if ye forgive men their trespasses, your heavenly Father will also forgive you: ¹⁵ But if ye forgive not men their trespasses, neither will your Father forgive your trespasses. (Matthew 6:14-15, KJV)

Confession of sins against God which were known only to God should not generally be made to humans. Humans cannot forgive us our sins, and their knowledge of them will only weaken them or tempt them in the same direction. It is like yeast--"a little leaven leaveneth the whole lump" (Galatians 5:9, KJV), to put it in Biblical terms.

Private sins should be confessed privately, and public sins--sins already known to the public--should be confessed just as publicly. But there is no need to confess sins known only to God to mortal ears; it will seldom, if ever, be productive of good.

All sins must be confessed to God, who only can forgive them; but God expects us to reconcile with our brethren, when we have offended them, before expecting His forgiveness. This is why the Bible tells us to confess our faults to one another. And Jesus also addressed this matter in his Sermon on the Mount:

²³ Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee; ²⁴ Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift. (Matthew 5:23-24, KJV)

Conclusion

Post-baptismal sins are forgiven in the same manner as pre-baptismal sins: by confession of our sins to God as outlined in 1 John 1:9. If our sins involved others, confession should be made also to them, to seek reconciliation with them, before we can expect God's forgiveness, as addressed in James 5:16, Matthew 5:23-24, and Matthew 6:14-15.

Protestants receive forgiveness for their post-baptismal sins in exactly the same way they received forgiveness for all their pre-baptismal sins. Upon sincere confession of their sinfulness, in prayer to God and through faith in the redemptive, finished work of Christ, God pardons them. Their conscience is cleansed as a gift of free grace, and the Christian then strives to keep that conscience clean before God. See Acts 24:16 & 2 Corinthians 1:12.

However, the question really asks how Protestants receive this forgiveness, as compared with the Catholic system of confession and absolution from sins. My opening paragraph shows the stark difference between a Catholic system which promises to deal with post-baptismal sins, and Protestants continuing to look to God, in Christ, for his promise of forgiveness to obtain till their dying days, as they continue to view sin the same way they did prior to their baptism. Nay! They view sin with increasing abhorrence as they grow in grace and Christian maturity.

You rightly said that "Protestant practices vary on this" matter of dealing with on-going sins, as there are different views. This answer seeks to give a Reformed Protestant view by (1) giving one biblical example; (2) one historic example at the Reformation; (3) quotes from modern Reformed doctrinal writings on the matter, which show the need to have a balanced view of the holiness of the Church, and Church discipline.

(1) Peter's three denials of Christ in one night:

After his baptism, Peter sinned against Christ on the night of his betrayal, despite Jesus warning him in advance that he would do that.

"No one should minimize the bravery of Peter, nor mistake his ardent devotion. Peter's tragic discovery, however, was that for all his determination not to deny Jesus, the flesh was helpless before the powers of darkness, nor could what was carnal attain to things that were spiritual... Satan prepared his sieve to sift Peter, and he hoped utterly to destroy his faith, bringing him down to despair and death... With all his spirit he had tried; and with all his flesh he had failed.

Nevertheless, Jesus had said beforehand, 'Simon, Simon, behold, Satan hath desired to have you, that he may sift you as wheat: but I have prayed for thee, that thy faith fail not: and when thou art converted [turned around], strengthen thy brethren (Luke 22:31-32). Mark John Metcalfe, pp 206-7

Note that all the other disciples had equally betrayed Jesus that night, by running away. They might not have denied him publicly, but at least Peter tried his hardest to remain as close as possible to Jesus as his ordeal began, for he had promised to Jesus that he would not forsake him. The others forsook him at the outset, so they were in no position to require Peter to confess to them, nor to offer him absolution! They all needed the offices of Christ as the only mediator between God and man. That Protestant view is shared by all.

(2) A Reformation example of dealing with post-baptismal sin:

"...the tensions which medieval religion deliberately induced, playing alternately upon fear and hope: Hell was stoked, not because men lived in perpetual dread, but precisely because they did not, and in order to instill enough fear to drive them to the sacraments of the Church. If they were petrified with terror, purgatory was introduced by way of mitigation as an intermediate place where those not bad enough for hell nor good enough for heaven might make further expiation. If this alleviation inspired complacency, the temperature was advanced on purgatory, and then the pressure was again relaxed through indulgences...

Like everyone else in the Middle Ages [Martin Luther] knew what to do about his plight. The Church taught that no sensible person would wait until his deathbed to make an act of contrition and plead for grace. From beginning to end the only secure course was to lay hold of every help the Church had to offer: sacraments, pilgrimages, indulgences, the intercession of the saints. Yet foolish was the man who relied solely on the good offices of the heavenly intercessors if he had done nothing to insure their favor!

And what better could he do than take the cowl? Men believed the end of the world already had been postponed for the sake of the Cistercian monks. Christ had just "bidden the angel blow his trumpet for the Last Judgment, when the Mother of Mercy fell at the feet of her Son and besought Him to spare awhile, 'at least for my friends of the Cistercian Order, that they may prepare themselves.' [Ref. Coulton 1, 92] ...

St. Thomas Aquinas himself declared the taking of the cowl to be second baptism, restoring the sinner to the state of innocence which he enjoyed when first baptized. The opinion was popular that if the monk should sin thereafter, he was peculiarly privileged because in his case repentance would bring restoration to the state of innocence. Monasticism was the way par excellence to heaven. Luther knew all this...

It was no wonder that after a visit with his parents, sudden lightning struck him to earth [and] he cried out to his father's saint, patroness of miners, "St. Anne help me! I will become a monk."...

"The whole sacramental system of the Church was designed to mediate to man God's help and favor. Particularly the sacrament of penance afforded solace, not to saints but to sinners. This only was required of them, that they should confess all their wrongdoing and seek absolution. Luther endeavored unremittingly to avail himself of this signal mercy. He confessed frequently, often daily, and for as long as six hours on a single occasion. Every sin in order to be absolved was to be confessed... Luther would review his entire life until the confessor grew weary and exclaimed, "Man, God is not angry with you. You are angry with God..." Staupitz [his confessor] said, "If you expect Christ to forgive you, come in with something to forgive - parricide, blasphemy, adultery - instead of all these peccadilloes." Here I Stand Roland Bainton pp28, 33, 54

So, when Luther took over the chair of Bible, which Staupitz once held, his study of the Bible (in order to teach it) revealed to him the biblical doctrines of sin, confession and forgiveness (especially as detailed in the book of Romans) and he was liberated into the freedom that Christ's redemption achieved. Not a freedom to sin but the opposite - living to be like Jesus in holiness and obedience. The sacerdotal system of Catholicism was abandoned.

(3) Modern Reformed doctrinal writings:

One example must suffice. This is about the Protestant's view of the holiness of the Church and its discipline to maintain that holiness. It shows that belonging to the Church Christ builds is essential - there can be no free-lance Christians, with nobody to answer to, here on earth.

"...even the regenerate members are 'holy and blameless' objectively in Christ alone and will not be subjectively complete in Christ until their glorification. This view of 'essential holiness' stands in some contrast with both Roman Catholic and Anabaptist/pietist alternatives. Both tend to lodge the church's holiness in some quality inherent in the institution or in the collective piety of its individual members. However, Scripture locates this attribute, like the others, in the triune God and his covenantal grace...

In contrast to the Roman Catholic paradigm of fusion, radical Protestant movements have tended to lodge the church's holiness in the identifiable experience of the many (regenerate believers) as opposed to the collective identity of the one church. In the Roman Catholic perspective, the church makes the saints, while in the radical Protestant view, the saints make the church. This draws a sharp contrast between the Spirit's sanctifying work within believers and the visible means of grace... Both views, however, tend to lodge the church's holiness within the inner condition of the church itself. This is where we find the most striking contrast with a covenantal ecclesiology [the Reformed position].

In a covenantal ecclesiology the church is holy because the covenant of grace is the sphere within which the Holy Trinity gathers, protects, and keeps a people set apart from the world...

Grounded in the proper preaching of the gospel and administration of the sacraments, discipline holds its proper place as a mark of the true church... This is part of the ministry of the key, taught by Jesus in Matthew 18, where the keys of the kingdom are given to the church's pastors." Pilgrim Theology Michael Horton, pp 404-406

Much more is explained, showing that Reformed Christians believe in the gathered church, which has nothing to do with denominations, but the work of the Holy Spirit in sanctifying repentant sinners and indwelling them to complete the good work begun in them, till Christ returns. They eagerly seek to become more and more Christ-like, for his glory, not for personal benefits. Already this answer is very long, and yet has barely scratched the surface of this deep subject, but I hope it will have outlined some important points upon which Reformed Christians have moved away from Catholic systems of confession and penance.

I know from my Lutheran days that Lutherans have a confiteor at the beginning of each service, and that they believe this confession (with repentant heart) is sufficient to bring about reconciliation with God for all sins. Similarly, Catholics hold that our confiteor offers forgiveness for any venial sins, whereas Sacramental Confession is required for forgiveness of post-baptismal mortal sins. Generally, Lutherans and many other kinds of Protestants make no distinction between mortal and venial, so it makes sense for Lutherans to believe that this confiteor is sufficient. Lutherans actually debate among themselves whether this common confiteor constitutes a third Sacrament, though the Missouri Synod, where I come from, currently does not formally teach that it is a Sacrament.

Lots of low-church Protestants believe that they just need to confess their sins in prayer and ask forgiveness, and that this can be done privately, or they may believe that just generally acknowledging their sinfulness in the abstract is sufficient. This latter view seems to be dangerous to me, since it doesn't appear to contain any implicit or explicit repentance. I also don't think that it's commonly taught, but rather that many behave as though they believe this.

Any "once saved always saved" Protestants are unconcerned with post-"baptismal" sins, since they don't believe that any sins could sever them from God's grace.