To whom ye forgive anything, I forgive also: for if I forgave anything, to whom i forgave it, for your sakes forgave I it, in the person of Christ; lest Satan should get advantage of us: for we are not ignorant of his devices (2corithians 2:10-11). In the Lord's prayer, Jesus taught that the condition for us to be forgiven by God is to forgive those that offend us.
There are many things God cannot do for you except you forgive those that offend you. Unforgiveness will put you in darkness. It gives the devil permission to operate in your life. Satan takes advantage of a heart that is filled with grudge. You must know that judgement belongs to God, do not try to take vengeance on anyone that offends you. Even as it is not wrong to correct someone that offends you, you must not hold the deeds against the person in your heart. Offences must come; you cannot prevent it, Luke 17:1. But you must leave all those that offend you to God, don't fight them by yourself or seek their destruction in your heart. Let God handle it.
The basis of your acceptance into God's family is forgiveness, for we all were dead and lost in sin until God by His mercy forgave mankind and brought about restoration achieved by the sacrifice of His son, Jesus on the cross. If God forgave you not minding your unrighteousness, then you also are duty bound to forgive all those that offend you. If you do not live a life of forgiveness, you will live in bondage and will be stagnated. Forgiveness on the other hand brings promotion. Despite all that Jesus suffered on the cross, He forgave all those that offended Him.
Unforgiveness disqualifies you from God's presence Hebrew 12:15. Thus those that wish to maintain an unbroken relationship with God must put away bitterness and an unforgiving spirit from their lives.
If you refuse to forgive, the sin that was covered will be re-opened. Devil does not have power, but he has devices and strategies. One of his strategies is to hinder you from forgiving those that have offended you. The unforgiving servant in the book of Matthew 18:32-35, was delivered to the tormentors. When you hold people responsible for unpleasant situations that happened to you, then you will find it difficult to forgive them. Forgiveness has no limit, you have to keep forgiving. Jesus told Peter to forgive his brother that has offended him seven times seventy times. This just goes to buttress the point that our forgiveness should be without limit.
Forgiveness enhances the effectiveness of your prayers whereas unforgiveness hinders the effectiveness of prayers. Mark 11:25- And when ye stand to pray forgive, if you have ought against any: that your father also which is in heaven may forgive you your trespasses. Forgiving those that offend have offended you is therefore to your own advantage, you are helping nobody but yourself.