What does Mark 11:26 mean?

This verse is not found in the more accurate ancient manuscripts. The New American Standard Version reads "But if you do not forgive, neither will your Father who is in heaven forgive your transgressions," but the verse is in brackets, showing that the translators don't think it's original. The sentiment is consistent with Scripture and found in Matthew 6:14–15, at the end of the Lord's Prayer, and briefly in Luke 6:37. Nowhere else does Mark use the terms "heavenly Father" for God or "transgressions" for sin, so in the translations where it does occur, it's probably copied from Matthew.

When Jesus says that if we do not forgive others we will not be forgiven by God, He does not mean we will lose or forfeit salvation if we do not fully forgive every single person who wronged us. In fact, the "forgiveness" mentioned here is not about eternal salvation, at all. Rather, this is relational forgiveness. The parallel for this would be offending or sinning against one's spouse, without asking for forgiveness. That would hurt a marriage relationship, and result in distance or separation, but not an utter end to that commitment.

Likewise, if we sin against God and do not ask for forgiveness, we do not lose our salvation. Our salvation is dependent on the work of Jesus, not our ability to remember every single sin and repent of them (Titus 3:5). However, sin does harm our relationship with God, interfering with our growth and His blessings. God designed us for community, honesty, and humility. If we sin against someone else without asking their forgiveness, we sin against God (Matthew 18:23–35). If we stubbornly refuse to forgive others, we're not reflecting appreciation for the forgiveness we, ourselves, have received (Mark 11:25).