Leviticus 19:18 KJV
Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the Lord.
Let us just break these words down: Thou shall Not avenge. There's no other way to make this clearer or its impact more pronounced than to establish the identity of the one making the statement. For every instruction God gives, there are blessings for obedience and punishment for disobedience. God never wastes words so you can rest assured that every word spoken by him has the backing of his name and he will make sure that they all come to pass.
What's the warning: Do Not Avenge. If we search our hearts sincerely, have we not been a bit Happy that people who did something bad to us met with a bad incident Sometimes later? We didn't pray that they should go through such, but we didn't pray that they shouldn't either. We need to keep God's searchlight over our hearts constantly so he can keep cleansing us of evil thoughts. God judges our intentions and motives and he cannot be deceived by the external.
You may say you do not avenge evil done towards you by others but what about hearing grudges? There's a silent poison that wants to pollute the church today and it is this silent malice or the superiority complex. Someone does something to us, and rather than call the person to understand the motives or intentions behind those actions, we presume we know why and we react accordingly. We play God, interpreting other people's intentions and motives, forgetting that this is God's prerogative. God goes a step further to underline the importance of Not bearing grudges in Matthew 5:23-24 KJV:
Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee; Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift.
When next you think that God appreciates your fat tithes, you will do well to remember that God accepted Abel first, before accepting his offering. He rejected Cain first, before rejecting his offering. Your life and eternity are much more important than what you own or hold now. Get your priorities right; his time of favour is here.