Forgiveness  Matthew 6:14, 15
"For if you  forgive men their trespasses, your heavenly Father will also forgive you, but if you do not forgive men their trespasses, nether will your Father forgive your trespasses." 

In Matthew 18:21 perhaps Peter was trying to get Jesus' permission to retaliate physically against someone creating a problem in his life when ge asked Jesus, "Lord, how often shall my brother sin against me and I forgive him?  Till 7 times?"  But Jesus says in verse 22, to only forgive 7 times is not the Christian way, but 70 times 7 (490) times is a better standard. 

In Matthew 18:35 Jesus repeats that it is extremely important to our spiritual life to forgive "everyone his brother their trespasses." 

When we look back on how many times God has forgiven us for thoughts, words, deeds and even things we should of done but didn't we realize that we have the same obligation to forgive others.