Forgiveness? Under whose authority? He has no more authority to forgive sins than do I.
http://blog.adw.org/2015/08/greek-to-you-dont-dismiss-it-the-importance-of-recourse-to-the-greek-text-of-the-new-testament/
Here's an excerpt:
Note that the verbs related to heaven’s binding and loosing are dedemenon and lelumenon. They are perfect (passive) participles in the middle voice. As such, they indicate something that has already been done in Heaven before Peter does it on Earth.
Hence a literal, though awkward, English rendering would be “Whatever you might bind on the earth, having (already) been bound in heaven, and whatever you might loose on the earth, having (already) been loosed in heaven.”
But this is just not the way we talk in English. And thus most English renderings go something like this: “Whatever you bind on earth will be bound in heaven and whatever you loose on earth will be loosed in heaven.” And, while smoother, it loses the inspirational emphasis that the Greek text conveys.
The Greek text makes clear that if Peter binds or looses something on Earth, it is because Heaven has inspired this act; in no way is Heaven engaged in a “rearguard action.” Rather, Peter is inspired to carry out what has already been done in Heaven. Heaven is not forced to comply with Peter’s decision. Rather, Heaven binds or looses, and then inspires Peter and his successors to do likewise. The Greek conveys this important subtlety; the English does not.