Consider the following verse related to Stephen's stoning:
Then he fell on his knees and cried out, "Lord, do not hold this sin against them." When he had said this, he fell asleep.
Clearly a Christian can forgive another person and, in fact, we are commanded to do so. But to wipe that sin off The Lord's books, my understanding is that's something only the individual sinner can do through true repentance via God's grace. The justification occurs between two parties: God and the sinner. The third-party victim of the sin can't act as a mediator.
Jesus proactively canceled others' sins on a number of occasions, most notably at the cross where He said, regarding his executioners, "Forgive them, Father. They know not what they do." But I had assumed that was something that only He could do; He's God, after all. But why was Stephen able to request that God forgive the sin of others? What authority did Stephen have to make that request of God and further, do we as Christians have that same authority?
Example: Let's say a non-believer is caught stealing $50 from you. You grant him forgiveness but he's still guilty of the sin before God. Do you then also have the authority to ask The Lord to forgive him for the theft, despite that he's a non-believer who doesn't fear God, nor feels any need of repentance whatsoever?
This question is being asked from a Protestant evangelical perspective.