Mark 11:25-26
25 And when ye stand praying, forgive, if ye have ought against any: that your Father also which is in heaven may forgive you your trespasses.
26 But if ye do not forgive, neither will your Father which is in heaven forgive your trespasses.
The Grace New Testament Commentary - Mark 11:25-26
11:25-26 Forgiveness is also important in prayer. When a believer is praying, God often brings people to his mind who have wronged him, and therefore "if you have anything against anyone, forgive him."
Forgiveness of others is necessary so "that your Father in heaven may also forgive you your trespasses." Our fellowship forgiveness with the Father is conditioned upon our forgiveness of others. Jesus emphasizes the negative aspect that "if you do not forgive (i.e., others), neither will your Father forgive your trespasses." As a child of God by faith alone in Christ alone, all of one's sins are forgiven forever (cf. Col 2:13-14), but his or her fellowship with God is conditioned on confession of one's sins as a believer (cf. 1 John 1:9). A Christian cannot have fellowship with the Father if he has not forgiven others.
Charles Bing - Grace, Salvation, and Discipleship - Forgive to be Forgiven, Matthew 6:14-15 (cf. Mark 11:25-26)
(Salvation) Interpretation: If a person does not forgive others who sin against him, he cannot be saved, or will lose his salvation.
(Discipleship) Interpretation: If a believer does not forgive others who sin against him, he will suffer in the quality of his fellowship with God.
The statement in Matthew is clear in its teaching: If someone does not forgive others, then God will not forgive him. If this forgiveness from God refers to the forgiveness that we receive at the time of salvation (Eph. 1:7; Col. 2:13-14), then it makes salvation conditioned on our willingness to forgive others. As a (Salvation) statement, it would speak of the positional aspect of our salvation: In God's eyes we are forgiven once and for all and forever based on the payment for our sins by Jesus Christ dying on the cross.
But there is another kind of forgiveness. In fact, the context demands it because here, Jesus is teaching His followers to pray (Matt. 6:5-13). This forgiveness is not positional forgiveness, but fellowship forgiveness because it tells those who already know God how to maintain a close walk with Him. Jesus taught this truth to Peter and the disciples in the Upper Room. When He attempted to wash Peter's feet, Peter refused Him. Jesus then explained, "He who is bathed needs only to wash his feet, but is completely clean; and you are clean, but not all of you." So for those who are saved, there is a daily need to confess sins and claim a new experience of fellowship forgiveness based on Christ's past provision (His death on the cross) and our present position (a child of God). 1 John 1:9 shows that confessing sin and receiving God's forgiveness is necessary for walking in the light and maintaining fellowship with God (1 John 1:3-8).
In the realm of discipleship, forgiveness is very important to Christians not only to maintain fellowship with God, but harmony with other people. Thus, we are exhorted to forgive others just as Jesus Christ forgave us (Eph. 4:32; Col. 3:13). If we harbor unforgiveness toward others, we will not enjoy the intimacy God desires to have with us. If we harbor unforgiveness toward others, not only do we disobey God's command to forgive, but the barrier of our hardened heart keeps us from enjoying God's full fellowship.
Understanding this truth about fellowship forgiveness keeps us from imposing a condition on the gospel other than faith alone. The gospel invitation is not, "Believe and forgive others," but simply "Believe."