Well, it is a start. You get credit for *doing something*, which is more than most humans. 😊
The second part of each conditional clause in this verse is in the passive voice and the perfect tense in the Greek text. The passive voice indicates that someone has already done the forgiving or retaining.That person must be God since He alone has the authority to do that (Matt. 9:2–3; Mark 2:7; Luke 5:21).
The perfect tense indicates that the action has continuing effects; the sins stand forgiven or retained at least temporarily if not permanently.
If people (“any” or “anyone,” plural Gr. tinon) believed the gospel, the disciples could tell the believers that God had forgiven their sins. If they disbelieved, they could tell them that God had not forgiven but retained their sins. Jesus had done this (cf. 9:39–41), and now His disciples would continue to do it.
One last thought for your dear friend, no where in Scripture do we find an Apostle remitting the sins of anyone. They proclaim the forgiveness of sins everywhere.
best
No, the passive voice means that the actions was done by someone other than the subject of the verb. And yes, that would be by God. But what is important here is the moods that are used in the two clauses: subjunctive/indicative. The action in the indicative is predicated on the action in the subjunctive being done. Thus the actions of forgiving/retaining [by God] is predicated on the forgiving/retaining by the Apostles. Nor does this take away that ultimately this is done by God himself. Rather, that God forgives/retains through the human agents to whom he has delegated this authority.
It should also be noted that the forgiving/retaining by God is not predicated on the proclaiming the forgiving/retaining but by the act of forgiving/retaining by the Apostles.