Why is withholding forgiveness wrong? Since God does not forgive a person until they confess/repent, can we withhold forgiveness until a person confesses/repents?

from the old testament

• Forgiveness is demonstrated between Jacob and Esau in Genesis 33:3-16. Despite a dark past between the two brothers in which Jacob was largely to blame, much to Jacob’s surprise Esau chooses to make peace with him instead of enacting war. Esau’s act of reconciliation is a profound example we can follow when it comes to forgiving someone who has deeply wronged us—especially within a family unit (verse 4).
• God’s forgiveness toward the Israelites’ sin is demonstrated numerous times in the Old Testament. For example, after they worshiped the golden calf, God both declared His forgiveness as well as affirmed His justice to Moses (Exodus 34:6-7, 9-10).

from the new testament

• Refusing to forgive is sin (Matthew 5:22–24). It comes from bitterness, resentment, and anger that is the natural result of being hurt. However, that is not an excuse not to forgive. We are supposed to forgive our enemies, and even pray for them (Matthew 5:44). The ability to forgive an enemy comes from God, "for he makes his sun rise on the evil and on the good, and sends rain on the just and on the unjust" (Matthew 5:45). From this, we see that God's heart is to forgive even those that are His enemies, and as God's children our hearts should reflect His.
• Though Matthew 18:15–17 does not specifically address forgiveness, it does address how to handle confrontation and reconciliation which often go in hand with forgiveness. If a believer has wronged us, we should confront him privately (verse 15). If he refuses to acknowledge his sin, we can then gather one or two trusted believers to join us (verse 16). If the situation cannot be solved at that point, then it should go to the church (verse 17). If the offender rejects the church’s efforts, he should be rejected by the entire church body until he repents (verse 17).
• Jesus illustrates His stance on forgiveness with a parable found in Matthew 18:23-35: A king forgives a servant’s debt, but the servant abuses someone who owes him a smaller debt than the servant owed the king. Hearing of this, the king threw the unforgiving servant in jail. At the end of the parable, Jesus said, "So also my heavenly Father will do to every one of you, if you do not forgive your brother from your heart" (Matthew 18:35).
• A wonderful depiction of forgiveness is the parable about the prodigal son. After a wayward son disrespects his father and makes a mess of his life, his father graciously and lovingly receives him back when he returns home (Luke 15:11-32). This is the type of forgiveness God extends to us for our sins against Him.
• God's nature is to forgive. Some of Jesus' last words from the cross were to forgive those who were crucifying Him because they didn't understand what they were doing (Luke 23:34).
• Forgiveness has tremendously blessed believers since we didn’t deserve God’s grace and love. As Ephesians 1:7-10 says, “In him we have redemption through his blood, the forgiveness of our trespasses, according to the riches of his grace, which he lavished upon us, in all wisdom and insight, making known to us the mystery of his will, according to his purpose, which he set forth in Christ as a plan for the fullness of time, to unite all things in Christ, things in heaven and things on earth in him.” We must extend forgiveness, grace, and love to others who don’t "deserve" it from us.

implications for today

God forgives those who repent (Acts 3:19). When people refuse to repent and reject the free gift of God, which is eternal life in Jesus Christ, He allows them the wages of their sin—death (Romans 6:23). As such, some people assume that we are allowed to withhold forgiveness from one another if repentance is not present. This belief is even more confounded in us if the pain caused is intentional, callous, or repeated by our offender, and we don’t see them express remorse at all. Yet even so, Christians are to be ready to forgive those who sin and then repent, even when the offense is committed multiple times (Matthew 6:14–15; 18:21–22; 18:23–35; Mark 11:25; Luke 17:3–4; Ephesians 4:31–32; Colossians 3:13). At the same time, we are called to forgive even if the person never repents, trusting that God is in control. That does not mean that we are friends with the person or that the relationship continues as it once was. However, it means we willingly give that person and the situation that caused us hurt to God, allowing God to work in our hearts instead of allowing bitterness to form and grow.

Jesus was brutalized, bore the guilt of our sins, and was murdered before we ever said sorry to God or repented. Jesus’ forgiveness saw past the cross, past the pain, and past our wrongs—He gave love instead of hate, grace instead of condemnation—and we should do the same for those who have sinned against us.