The perfect tense generally indicates a continuous action - the apostles are tasked with continuing to proclaim what Jesus began - that if one puts their faith in Him, their sins are forgiven. If they don't, their sins aren't forgiven. That's the option. Believe and be forgiven; don't believe and you aren't forgiven - as noted in John 3:36 which both 1 Thess and I quoted before.Thanks for your thoughtful response, Illini.
A couple of things. Doesn't the past perfect tense denote an action that is completed not that "occurs in the past and continues into the present" (is that the past aorist tense?). I agree that: "If you forgive sins of anyone, they have been forgiven; if you retain sins of anyone, they have been retained", is a good translation. However, isn't "because of their faith/unbelief" a theological interpolation and not actually in the text? Also, the verb directed to the disciples is in the subjunctive, which means they have the option of forgiving/not forgiving; however, if it's just about faith, how can they possibly have the option? They don't control the faith of people.
I see what you mean about the disciples going out to do the ministry of Jesus. But Jesus forgave sins. Furthermore, the pericope doesn't specifically mention preaching, just going out as Jesus did.
That's the whole point of salvation.
Jesus didn't give any part of His Deity (which, forgiving sins would be included there since no one but God can do that) to anyone. He didn't give them authority to forgive sins, He gave them the authority to proclaim what He already did.