Given that forgiveness is literally the main theme of our religion, there should never be a merciless Christian. All of our sins, past present and future, have been nailed to a cross, and when our Saviour exited His grave, our sins remained there. How angry could God be with you right now? Even if you have only been awake one hour, I bet most of you could specifically name a sin you have committed today. If not, look at the last 24 hours. You failed yesterday, you fail today, you will fail tomorrow, and yet, God does not hold this against us. He held it against Jesus instead.
So what right have we to count another's sins? Who are we to withhold forgiveness? How, when we look at a fellow object of God's mercy, can we continue to cling to wrath? Is it not God's alone to avenge? Can you legitimately overthrow God's judgements? If you must deal with another's sin, do it in order to edify them. Let God worry about the rest.