The Heidelberg Catechism: Lord's Day 51
Question 126: What does the fifth request mean?
Answer: Forgive us our debts, as we also have forgiven our debtors means,
Because of Christ’s blood, do not hold against us, poor sinners that we are, any of the sins we do or the evil that constantly clings to us. 
Forgive us just as we are fully determined, as evidence of your grace in us, to forgive our neighbors. 
 Ps. 51:1-7; 143:2; Rom. 8:1; 1 John 2:1-2;  Matt. 6:14-15; 18:21-35
The Heidelberg Catechism
Lord's Day 1