There is one tell-tale fruit, but it may take a long time for it to happen. And even then you likely won’t see it. But here’s the fruit nonetheless — if the sinner ends up in heaven, you will know they had truly repented. If not, they likely had not. I understand the desire to know the sincerity of another’s repentance. I’ve been in countless pastoral situations wherein it seemed like the answer to that one question — is this person truly repentant — determined the answer to every other question about what should be done. Trouble is, God has not been pleased to give us the means to peer into the souls of others.
So what do we do? Consider the case of adultery, an all too common grievous sin. Suppose I am unfaithful to my wife. Suppose I claim to be repentant. What ought she to do? The Bible says that she is free to divorce me, but is not required to do so. Many times her decision is bound up in this question — is he repentant? But that’s not really the question. If I am repentant, her duty is to forgive me. But her duty is not to remain married to me. If I am feigning repentance, and she decides to stay with me, but later determines my repentance isn’t sincere, even if I so confess, she is not free to divorce me. That’s why my counsel in these circumstances is to encourage thinking through this question- would you, knowing what you now know, marry this person? If not, forgive and divorce. If so, forgive and stay together. But you don’t need to know if the repentance is sincere.
One parenthetical thought. I consider it good evidence, though not compelling proof, that a person is sincere in their repentance if they repent before their offense is known, and if they repent of what would otherwise never be known. Such doesn’t mean, on the other hand, that only this kind of repentance is sincere. David was busted by Nathan before he came to repentance. But I doubt anyone would doubt his sincerity after reading Psalm 51.
A second parenthetical thought. Remember that repentance doesn’t undo sin. It merely acknowledges that it can’t be undone. In like manner, repentance doesn’t atone for sin. Jesus does. The inescapable conclusion is, how much repentance is “enough?” It’s never enough. All of us, no matter how deeply we might repent, ought to repent for the shallowness of our repentance.
The hope that time will tell is elusive. The unrepentant can appear repentant for a long time. The repentant, on the other hand, sin all the time, making it all too easy to doubt their repentance. In the end, therefore, all we are left to do is to exercise our best judgment, and I would argue, to practice a judgment of charity. Perhaps the best indicator I know of is this — is the sinner owning their sin, and standing ready to do whatever is necessary to make right, as much as is possible what they have done. Which is to say, the repentant are those who repent. Can the unrepentant fake this? Yes, but usually they do not.
We cannot go through our lives afraid that we might forgive the unrepentant. We ought to go through our lives afraid we have failed to forgive the repentant. With the former we may allow ourselves to be wronged, with the latter we are wronging others.