Hypothetical situation: A friend, loved one, coworker, or acquaintance has a certain behavioral trait, one that is objectively sinful and hurtful. At times, you are the one s/he hurts. Every time s/he hurts you, s/he apologizes. After n apologies and n +1 times being hurt, though, don’t you have a right to feel his/her apologies are insincere? Shouldn’t the behavior have been corrected by then if s/he really meant it? In sum, aren’t you justified in refusing to forgive, or making your forgiveness contingent upon some material act?
Seventy Times Seven
There are only two passages in the New Testament where a finite number is connected with forgiveness. The first occurs in Matthew 18:21-22:
Then Peter came up and said to him, “Lord, how often shall my brother sin against me, and I forgive him? As many as seven times?” Jesus said to him, “I do not say to you seven times, but seventy times seven.”
Jesus then follows this injunction with the parable of the Unforgiving Servant (vv. 23-35). The “seventy times seven”, of course, is hyperbole meaning that we forgive as often as asked: “... and if [your brother] sins against you seven times in the day, and turns to you seven times, and says, ‘I repent,’ you must forgive him” (Luke 17:4). However, you don’t have to wait to be asked forgiveness in order to grant it.
As for sincerity — nope. Don’t find it connected to forgiveness of others anywhere in the NT. It’s not a condition. Nor do you find any passage that allows you to make forgiveness conditional. Catholics have done penitential acts over the centuries. However, those acts were reparative; that is, they were ordered towards repairing the relationship between the person and God, and not as a condition of His forgiveness. Making your forgiveness contingent upon fulfilling a material condition as “proof” of sincerity is spiritual extortion.