Matthew 18 provides a lot of enlightenment. First off, it clarifies what the "unforgivable sin" is -
3 And he said: “Truly I tell you, unless you change and become like little children, you will never enter the kingdom of heaven.
The sin against the Holy Spirit is the rejection of The Spirit. This is in spite of being baptized or Christ's sacrifice on the cross.
Second, it sheds light on the Church being an institution, not an individual thing.
15 “If your brother or sister sins, go and point out their fault, just between the two of you. If they listen to you, you have won them over.
16 But if they will not listen, take one or two others along, so that ‘every matter may be established by the testimony of two or three witnesses.’
17 If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector.
Also notice that this is in spite of any baptism of the offender or Christ's passion.
Then, along with 15-17, 18 illuminates some of the authority that not only goes along with the keys given to Peter in Matthew 16, but the priesthood in the order of Melchezidek. Taken with 15-17, you see this is the authority to mediate sins.
18 “Truly I tell you, whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.
19 I will give you the keys of the kingdom of heaven; whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.”
