It seems to me that one of the most difficult things that God asks us to do is to forgive. And yet God sets it to us straight when we are told in Matthew 6:12 in the Lord's prayer, "And forgive us our debts, as we also have forgiven our debtors." The "as" is important, because it tells us that God forgives our sins based on the way we forgive others. And if we haven't gotten it by that, Jesus says it again 2 verses later: "For if you forgive others their trespasses, your heavenly Father will also forgive you; but if you do not forgive others, neither will your Father forgive your trespasses." Pretty strong words for us to think about. And my first reaction to this is like what my children often say to me: "That's not fair!" We think it's not fair because we think that we if we have faith in Jesus, then we should receive the forgiveness of Jesus that He talks about.
So how is it that our being forgiven is tied into how we forgive others? Let me list a couple of reasons. First, it is for reconciliation. God desires for us to find reconciliation with others. Just like a parent doesn't want their children to be at odds with each other, so God does not want us to be at odds with each other. So, through forgiveness we can seek to be reconciled with each other. And the important factor here is that we can't determine how anyone else will act. Our role is to seek to be at peace with all (Romans 12:18).
Second, it is for our own well being. You might have heard of the saying, "Forgiveness sets you free." This is very true. God knows that when we don't forgive, we harbor feelings of pain, hurt, anger, frustration, bitterness, and on and on it goes. Having an unforgiving heart can tear us up inside. To forgive another person isn't saying that what they did is okay, it is saying that you won't hold on to it, you will seek to not remember it, you will not allow it to control your mind and your emotions. By forgiving the other person you are setting yourself free. Jesus Himself, while on the cross forgave those who were crucifying Him with the words, "Father, forgive them, for they do not know what they are doing."
Third, it is for our own understanding. If we don't know how to forgive, then we won't know how to receive God's forgiveness. We learn by experiencing. And when we experience what it feels like to forgive another, when we experience the strength it takes to forgive another, then we begin to understand what God goes through every time God forgives us. Our forgiving heart helps us to be thankful for the forgiveness God so readily offers to us: "If you confess your sins, God is faithful and just to forgive your sins and to cleanse you from all unrighteousness." (1 John 1:9)