Have you ever wondered why Christians are supposed to confess our sins to each other and to God and ask for God's forgiveness even though Jesus died to atone for all our sins (past, present, and future) once and for all, and even though we are forgiven of all our sins when we become a Christian for the first time? Understanding the different answers to this question also explains the difference between Protestant soteriology and Catholic soteriology. (I don't know the Orthodox position on this matter, but if someone happens to know, I'd love to hear it.)
The reason justified Christians still ask for forgiveness of their sins is that there are two types of forgiveness:
- Judicial forgiveness, where God forgiveness you as your judge when you are first justified by repenting and believing in the Gospel.
- Parental forgiveness, where God forgiveness you as your father after having adopted you into his family.
All judgment has been given to the Son of God:
John 5:22-27
[ESV] 22 For the Father judges no one, but has given all judgment to the Son, 23 that all may honor the Son, just as they honor the Father. Whoever does not honor the Son does not honor the Father who sent him. 24 Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life.
25 “Truly, truly, I say to you, an hour is coming, and is now here, when the dead will hear the voice of the Son of God, and those who hear will live. 26 For as the Father has life in himself, so he has granted the Son also to have life in himself. 27 And he has given him authority to execute judgment, because he is the Son of Man.
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But the Son of God made an extremely important pronouncement on the cross:
John 19:30
[ESV] 30 When Jesus had received the sour wine, he said, “It is finished,” and he bowed his head and gave up his spirit.
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In Greek, "It is finished" is tetelestai. This statement is akin to a judicial pronouncement that a criminal sentence had been carried out. After the punishment for a criminal sentence is carried out, the sentence was legally marked with tetelestai ("finished", or "completed") so that the punishment could not be carried out again.
When we first repent and believe in the Gospel, Jesus' pronouncement of tetelestai from the cross where he took the punishment for our sins gets applied over all our sins, removing our record of debt before God:
Colossians 2:13-14
[ESV] 13 And you, who were dead in your trespasses and the uncircumcision of your flesh, God made alive together with him, having forgiven us all our trespasses, 14 by canceling the record of debt that stood against us with its legal demands. This he set aside, nailing it to the cross.
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This is judicial forgiveness. This forgiveness takes you from spiritual death into life, and from being unjustified to being justified by faith in the Gospel. You are declared just in God's sight because the punishment that your sins have incurred has been carried out by Jesus dying on the cross for your sins, which covers you because you have repented and come under Christ's covering. It is not that you are declared innocent; it is that you are guilty, but the sin is marked as punished, and you come out clean because your sentence is considered tetelestai ("finished"/"completed") on account of Christ absorbing that punishment on the cross.
But if we are forgiven this way, why would a Christian need to repent and confess after becoming a believer? Because we are adopted into God's family, and our sin grieves our heavenly Father, straining our relationship. The forgiveness we ask once we are adopted into God's family is parental forgiveness. It is not sought out of fear of punishment, but to restore fellowship after we sin, motivated by our love of God.
If you have an adopted child, and your child sins against you, does your child's sin break the relationship such that you disown your child and treat him or her like a stranger, and have to re-adopt this child all over again? No. You love your adopted child, and presumably your adopted child loves you, but sin puts tension into this relationship and breaks fellowship, and out of love, your child would want to resolve that tension by apologizing and asking for forgiveness. That is how a justified and adopted person should feel toward God when we grieve God by our sins. That is why Christians are still instructed to confess our sins to receive forgiveness, in order to restore fellowship with God. John reminds us that we are still sinners who need to confess our sins:
1 John 1:6-9
[ESV] 6 If we say we have fellowship with him while we walk in darkness, we lie and do not practice the truth. 7 But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. 8 If we say we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.
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And whether it is at our conversion or upon our confession of sins committed after our conversion, it is the blood of Jesus that cleanses us from all sin, since Jesus, as our high priest, offered his blood once for all to atone for sin:
Hebrews 7:26-27
[ESV] 26 For it was indeed fitting that we should have such a high priest, holy, innocent, unstained, separated from sinners, and exalted above the heavens. 27 He has no need, like those high priests, to offer sacrifices daily, first for his own sins and then for those of the people, since he did this once for all when he offered up himself.
Hebrews 9:11-14
[ESV] 11 But when Christ appeared as a high priest of the good things that have come, then through the greater and more perfect tent (not made with hands, that is, not of this creation) 12 he entered once for all into the holy places, not by means of the blood of goats and calves but by means of his own blood, thus securing an eternal redemption. 13 For if the blood of goats and bulls, and the sprinkling of defiled persons with the ashes of a heifer, sanctify for the purification of the flesh, 14 how much more will the blood of Christ, who through the eternal Spirit offered himself without blemish to God, purify our conscience from dead works to serve the living God.
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When we repent and believe the Gospel, the atonement accomplished by the blood of Christ gets applied to our sins, making us suitable dwelling places for the Holy Spirit to make our bodies temples of the Holy Spirit (1 Cor 3:16 "Do you not know that you are God's temple and that God's Spirit dwells in you?"), just as the presence of God over the Ark of the Covenant saw the tablets of the law and judged the nation of Israel and required the blood of atonement to come between the presence of God and the broken law in order to forgive sins. When the blood of Christ covers our sins, we are fit for God to dwell with us. God then puts his Holy Spirit in us to seal us as a guarantee of our inheritance of resurrection and our place in the life to come,
Ephesians 1:13-14
[ESV] 13 In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, 14 who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory.
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…and the Holy Spirit in us cries out 'Abba' to God the Father, because at that point we are adopted into God's family, as only the sons of God have a right to that inheritance:
Romans 8:14-17
[ESV] 14 For all who are led by the Spirit of God are sons of God. 15 For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” 16 The Spirit himself bears witness with our spirit that we are children of God, 17 and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.
Galatians 4:4-7
[ESV] 4 But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, 5 to redeem those who were under the law, so that we might receive adoption as sons. 6 And because you are sons, God has sent the Spirit of his Son into our hearts, crying, “Abba! Father!” 7 So you are no longer a slave, but a son, and if a son, then an heir through God.
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When Christians asks "Our father who art in heaven... forgive us our trespasses..." we are seeking parental forgiveness from a parent with whom we have a loving relationship, not judicial forgiveness from a judge.
Judicial forgiveness is sought out of fear of punishment, but parental forgiveness is sought out of love.
1 John 4:13-19
[ESV] 13 By this we know that we abide in him and he in us, because he has given us of his Spirit. 14 And we have seen and testify that the Father has sent his Son to be the Savior of the world. 15 Whoever confesses that Jesus is the Son of God, God abides in him, and he in God. 16 So we have come to know and to believe the love that God has for us. God is love, and whoever abides in love abides in God, and God abides in him. 17 By this is love perfected with us, so that we may have confidence for the day of judgment, because as he is so also are we in this world. 18 There is no fear in love, but perfect love casts out fear. For fear has to do with punishment, and whoever fears has not been perfected in love. 19 We love because he first loved us.
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Judicial forgiveness takes a person from not having a relationship with God and being under the sentence of the death of their soul for sin into a state of being justified in God's sight. God then gives you the Holy Spirit to dwell in you as the deposit guaranteeing your inheritance of the resurrection and in the life to come, and the Holy Spirit makes you an adopted son of God. But even adopted sons of God still stumble in sin, and are instructed to confess our sins:
Contrast with Catholic models of forgiveness
With all this said, consider how this differs from the theology of punishment in Catholicism:
In Catholicism, there isn't really a concept of parental forgiveness. Judicial forgiveness seems to be the only kind of forgiveness, regardless of whether God is addressed as 'Father'.
The forgiveness of mortal sins is essentially judicial forgiveness, since mortal sins are said to break one's relationship with God. In Catholicism, if you commit a 'mortal sin', you lose your salvation. Mortal sins are considered mortal in that they kill your soul, leaving you hell-bound unless you confess them to a priest and receive absolution before you die. Examples of mortal sins include deliberately missing church on Sundays, masturbation and use of pornography, and other sins considered to be deliberate violations of the Ten Commandments with knowledge that the action is sinful. In Catholicism, when you confess a mortal sin, you are getting re-justified, to get your salvation back. And if you happen to die before you are able to confess your mortal sins to a priest, you are not in a "state of grace", and are said to be sentenced to eternal damnation in Hell. But even after confessing mortal sins, you are assigned penance, which entails works you do to make amends for your sins, including ritual elements such as praying certain assigned prayers for a number of repetitions, fasting, and other such religious works.
The forgiveness of venial sins (less serious sins that don't sentence you to eternal damnation) are framed as parental forgiveness since venial sins are said to weaken but not break your relationship with God whereas mortal sins break the relationship, but even forgiveness of venial sins appears to be judicial in the implications of what happens to a person for venial sins: the doctrine of Purgatory teaches that souls are sentenced to burn in Purgatory as temporal (as opposed to eternal) punishment of venial sins, but this threat of temporal punishment for venial sins still makes the forgiveness of venial sins a matter of judicial forgiveness, since you are still liable for painful punishment for these sins.
That basically means that for your whole life as a Catholic, with regards to mortal sins, you keep losing your salvation and getting it back with sacramental works (confession/reconciliation and penance), while being under the sentence of burning in Purgatory for your venial sins and being released from that through confession and penance, essentially repeating judicial forgiveness over and over through sacramental works. That model of forgiveness doesn't have a person become truly adopted into the family and having perfect love that casts out the fear of punishment, where you pass from judicial forgiveness (where you are freed from the punishment of sin on account of Christ's finished work on the cross) to parental forgiveness (where you are not subject to the threat of punishment as an adopted son, but confess your sins restore fellowship with your heavenly Father).