So will my heavenly Father do to you unless each of you forgives your brother from your heart. The parable of the unforgiving servant of which read in today’s Gospel passage was given by Jesus in response to Peter’s question about forgiveness. It was a question that came in the backdrop of Jesus’ teaching about forgiveness (cf. Matthew 18:15-20). It would appear that Jesus had raised the bar rather high by requiring the offended party to go to great lengths to ensure that the offender is reconciled to the community. It was definitely a teaching that was new and putting it to practice was going to require some “putting in more hours.” This added responsibility was perhaps the reason why Peter wanted to know the number of times he was required by law to do this. In his response, Jesus not only tells Peter that forgiveness must be unlimited, but he also tells him that it must be authentic. 
To illustrate his point, Jesus differentiates between two types of forgiveness: forgiveness (from the mind), and forgiveness from the heart. There is forgiveness that can be theoretical, a forgiveness that only requires, so to speak, an assent of the "mind." It is a forgiveness that is merely a "noun," something that is out there and that is not part of me, something that I can pick and drop whenever I feel like it. Such is forgiveness that is extended as a matter of formality. The second type of forgiveness is practical, forgiveness that is more of a verb than a noun. This second type of forgiveness involves more than a mere assent of the will. It is something that I live. It is a verb, something that I do. It is not something that is apart from me but rather something that is part of me. It is a manner of forgiveness in which something is given up. This is forgiveness  from the heart and is the one that is authentic .
Jesus qualifies our requirement to forgive those who trespass against us by adding that it has to be forgiveness from the heart. Our forgiveness takes after God's forgiving us. The forgiveness that I extend to my brother is a reciprocation of God forgiving me. But unless I forgive, I cannot understand God's forgiveness. Unless I forgive my brothers and sisters from the depths of my heart, I cannot possibly be moved to appreciate God forgiving my trespasses. The servant in the story refused to forgive his fellow servant because he was yet to understand the essence of forgiveness: God forgiving us. He could only understand forgiveness by forgiving. He had to be involved in the very act of forgiveness in order to understand what the master had done to him.