After Jesus taught us what we now call the Lord’s Prayer, the first comment he made was:
For if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses. (Matthew 6:14-16 ESV)
The reason he did was because part of the Lord’s Prayer states:
“and forgive us our debts, as we also have forgiven our debtors”. (Matthew 6:12 ESV)
The words Jesus uses are interesting. He assumes that we have and will forgive those who are in our debt. But what does it mean to forgive others? Andrée Seu Peterson explains in very concrete terms: “Give your brother a clean slate every day” because we do not know how God is working with that individual. If someone wrongs us we are to forget that transgressions; remove it as far as the east is from the west. (Psalm 103:12) Christianity is to be positive and hopeful–“Love. . .believes all things, hopes all things” (1 Corinthians 13:7 ESV). 
Do we really forgive others their trespasses against us? Jesus’ statement in verses 14-16 is a warning that if we do not then God will not forgive us our trespasses. If our sins will not be forgiven, then how can we live in the presence of God? Our Christian doctrine tells us that sin separates us from God. The whole purpose of Jesus death and resurrection for our sins was to obtain forgiveness of our sins without which, our doctrine tells us, we would be separated from God for eternity.
This passage makes it very clear that God expects more of us than just our belief in Jesus and his death for our sins. God expects that we take action and in this passage God tells us one action he expects us to take is to forgive others their trespass against us.
 Andrée Seu Peterson, “Forgive those debtors”, World Magazine, October 15, 2016, p. 63.