Matthew 6 does not teach that our eternal destiny is based on our forgiving other people; however, it does teach that our relationship with God will be damaged if we refuse to pardon those who have offended us. The Bible is clear that God pardons sin by His grace based on Christ’s work on the cross alone, not on man’s actions. Our right standing before Him is established on one thing only—the finished work of Christ (John 3:16; 1 John 2:2; 1 John 4:10). The penalty for the sin that is rightly ours is paid by Christ, and we obtain it by grace through faith, not by any righteous deeds of our own (Ephesians 2:8-9). No one will be able to stand before God demanding that his sins be forgotten simply because he has forgiven others. Only when we are born again and given a new life through God’s Spirit by faith in Jesus Christ are our sins forgiven. Therefore, Jesus is not referring to God’s initial act of forgiveness (reconciliation) that we experienced when we first believed the Gospel.
What He is referring to is the day-to-day cleansing we obtain when we confess our sins in order to restore fellowship with our heavenly Father—the fellowship which is interrupted by the daily tarnishing of sin that affects us all. This is not the wholesale cleansing from sin that comes with salvation by grace through faith, but is more like the foot-washing Jesus describes in John 13:10. The “whole body is clean,” He told the disciples, but their feet were dirty from their walking in the world. Forgiveness in this sense is what God threatens to withhold from Christians who refuse to forgive others.
In Matthew 6 Jesus is teaching disciples how to pray and in doing so outlines how we are restored into intimacy with God whenever we have displeased Him. In fact, Jesus instructs us to build into our prayers a request for God to forgive us in the same way that we have forgiven others who have harmed us (Matthew 6:12). If there are those we have not forgiven when we ourselves pray for forgiveness, then practically speaking we are asking God not to restore a right relationship with us after we sin. To emphasize the importance of restoring broken relationships with our brothers and sisters, Jesus states that asking for God’s forgiveness for one’s own sins, all the while withholding forgiveness from someone else, is not only bizarre but hypocritical. We cannot possibly walk with God in true fellowship if we refuse to forgive others.
To be sure, an unforgiving spirit is a serious sin and should be confessed to God. If we have unforgiveness in our hearts against someone else, then we are acting in a way that is not pleasing to God, making our prayers and a proper living relationship with Him difficult. God will not hear our prayers unless we also show ourselves ready to grant forgiveness. To quote John Calvin on this verse, “If we are not harder than iron, this exhortation ought to soften us, and render us disposed to forgive offenses” (Commentary on Matthew, Mark, and Luke, Vol. 1).
A second biblically plausible interpretation of Matthew 6:14-15 is that it is saying anyone who refuses to forgive others is demonstrating that he has not truly received Christ’s forgiveness himself. Any sin committed against us, no matter how terrible, is trivial in comparison to our sins against God. If God has forgiven us of so much, how could we refuse to forgive others of so "little"? Matthew 6:14-15, according to this view, proclaims that anyone who harbors unforgiveness against others has not truly experienced God’s forgiveness. Both interpretations strongly deny that salvation is dependent on our forgiving others. Whether Matthew 6:14-15 is speaking of "relational forgiveness," or whether it is a declaration that unforgiveness is the mark of an unbeliever, the core truth is the same. We should forgive others because God, through Christ, has forgiven us (Ephesians 4:32). It is wrong for someone who has truly experienced God’s forgiveness to refuse to grant forgiveness to others.