{Whosesoever sins ye forgive} (an tin(936e) aph(8874)e tas
hamartias). "If the sins of any ye forgive" (aph(8874)e, second
aorist active subjunctive with an in the sense of ean), a
condition of the third class. Precisely so with "retain"
(krat(8874)e, present active subjunctive of krate(935c)). {They are
forgiven} (aphe(936e)tai). Perfect passive indicative of aphi(886d)i,
Doric perfect for apheintai. {Are retained} (kekrat(886e)tai).
Perfect passive indicative of krate(935c). The power to forgive sin
belongs only to God, but Jesus claimed to have this power and
right ( Mr 2:5-7 ). What he commits to the disciples and to us is
the power and privilege of giving assurance of the forgiveness of
sins by God by correctly announcing the terms of forgiveness.
There is no proof that he actually transferred to the apostles or
their successors the power in and of themselves to forgive sins.
In  Mt 16:19  18:18  we have a similar use of the rabbinical
metaphor of binding and loosing by proclaiming and teaching.
Jesus put into the hands of Peter and of all believers the keys
of the Kingdom which we should use to open the door for those who
wish to enter. This glorious promise applies to all believers who
will tell the story of Christ's love for men.