When Christians fall from the faith they show their true merciless condition. Think of Cain, Saul and Judas. This does not denote amount but spirit and attitude. Scholars have computed that the wicked servant was forgiven 600,000 times more than what was owed by the fellow servant. God has forgiven us infinitely more than our neighbor forgives us.
Mercy is an attitude of love and forgiveness toward someone who is miserable and undeserving.
His mercy endures forever. Holy Baptism is a constant and enduring covenant of God's mercy which forgives our many, many sins. In vs. 21 Jesus is speaking about a repentant sinner, a person who commits many sins of weakness. 15-18 Jesus had spoken of an impenitent sinner. Neither God nor the Christian should forgive an impenitent sinner.