Catechism 20: By What Authority Can A Priest Forgive Sin?
And again he entered into Capharnaum after some days. And it was heard that he was in the house, and many came together, so that there was no room; no, not even at the door; and he spoke to them the word.
And they came to him, bringing one sick of the palsy, who was carried by four. And when they could not offer him unto him for the multitude, they uncovered the roof where he was; and opening it, they let down the bed wherein the man sick of the palsy lay.
And when Jesus had seen their faith, he saith to the sick of the palsy: Son, thy sins are forgiven thee. And there were some of the scribes sitting there, and thinking in their hearts: Why doth this man speak thus? he blasphemeth. Who can forgive sins, but God only? Which Jesus presently knowing in his spirit, that they so thought within themselves, saith to them: Why think you these things in your hearts?
Which is easier, to say to the sick of the palsy: Thy sins are forgiven thee; or to say: Arise, take up thy bed, and walk? But that you may know that the Son of man hath power on earth to forgive sins, (he saith to the sick of the palsy,) I say to thee: Arise, take up thy bed, and go into thy house. And immediately he arose; and taking up his bed, went his way in the sight of all; so that all wondered and glorified God, saying: We never saw the like. (Mark 2:1-12)
Some friends of a man sick of palsy carried him to the house where Jesus was staying. Now to be sick with palsy was to be paralyzed either completely or partially. It could be accompanied by loss of sensation and uncontrollable body movements or tremors.
When these friends got to the house, they found the house to be packed with people wanting to hear Jesus and be healed by him. The friends climbed up an outside staircase and dug a hole through the roof. Then they lowered their friend down to the place where Jesus was. Jesus was so impressed by the faith of these men. First, Jesus forgave the man’s sins. and then, he healed the man so that he could get up and walk.
Now listen to what the scribes thought in their heart: "Why doth this man speak thus? he blasphemeth. Who can forgive sins, but God only?" (Mark 2:7)
First, We see that they considered Jesus as Man. They would be right. But that is only half the Truth. He was also God. The Man God.
Second, They felt that He was blaspheming. In this they would have been right except that they did not realize the whole story.
Thirdly, They were saying only God could forgive sin.
Were they right? Could only God forgive sin? The answer....yes. Ask just about any Protestant and they will say the same thing. Only God can forgive sin.
But listen to the answer Jesus gave when he realized what they were thinking:
"Which is easier, to say to the sick of the palsy: Thy sins are forgiven thee; or to say: Arise, take up thy bed, and walk? But that you may know that the Son of man hath power on earth to forgive sins, (he saith to the sick of the palsy,) I say to thee: Arise, take up thy bed, and go into thy house." (Mark 2: 10-11)
Let's look closer at Jesus answer.
He called Himself the Son of Man not the Son of God. Why?
The common understanding is that "Son of God" implies his deity—which it does—and that "Son of Man" implies his humanity, which it does too.
He was a son of man, that is, a human being. And He is the Son of God, in that He has always existed as the Eternally Begotten One who comes forth from the Father forever. He always has, and He always will. He is the Second Person of the Trinity with all of the divine nature fully in Him.
He is born of a Virgin. He was conceived of the Holy Spirit in the Virgin Mary. Thus He is human—fully human. The Bible wants to emphasize that He is fully human.
So that's the common understanding: He is both divine and He is human—two natures, one person.
If you do a study of the term "Son of Man" in the Gospels you'll see that he didn't refer to himself most often as Son of God but as Son of Man. He said things like, in Mark 10:45, "For the Son of man also is not come to be ministered unto, but to minister, and to give his life a redemption for many. ." So he calls himself Son of Man very often.
The Son of Man (Jesus in his humanity) had power.
What do we mean by having power? Was he saying he was a Super...Man?
There are six Greek words used for power in the New Testament. Exousia, dunamis, ischus, kratos, energeia, and bia.
For the sake of time we will only give the definition to the greek word that is used in Mark 2:10-11. And that word is Exousia.
Exousia: it denotes "authority", permission, or liberty of doing as one pleases, it passed to that of "the ability or strength with which one is endued," then to that of the "power of authority," the right to exercise power.
So we see that the Son of Man, (Jesus in his humanity), has power (has authority).
Who gave him that authority? Well, as the scribes said. "Only God can forgive sin" And who is God? As we said before, Jesus is God and Man and so He gave himself the authority, as God, to forgive sins.
Now lets fast forward to Jesus Ascension into heaven.
At this time He gives the great commission. Matthew, Mark and Luke each has a record of the great commission even though they differ in exact language (Matthew 28:18-19; Mark 16:-15-18; Luke 24:47-49). John 20:21-23 is John’s account of the great commission.
Matthew 28:18-19 records that Jesus said to the Apostles, "And Jesus coming, spoke to them, saying: All power is given to me in heaven and in earth." That word power is all Exousia, all authority. He therefore hands that authority over to the Apostles. "Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost. Teaching them to observe all things whatsoever I have commanded you.
Jesus authorizes, he exousia, the Apostles to make disciples, baptize, and catechise. All the Gospels pretty much say the same thing. But St. John's gospel added one more thing. After breathing on them and giving them the Holy Spirit, he says this:
Whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained. (John 20:23)
Remember when I said Only God can forgive sin? And because Jesus is God and Man He was able to give himself the authority, as God, to forgive sins on earth while he was in His body. But now at the ascension he is about to leave. He is leaving his body the Church (see I Corinthians 12) behind with the authority to do what He did as Man. And one of those things he gave authority to do is to forgive sins, or retain them.
Let me put it this way. In the Gospels, (Matthew 17:2, Mark 9:2–3, Luke 9:28–36), Jesus gave the Apostles a preview of what he would look like in his heavenly body during the Transfiguration. I believe that at the ascension they saw again the transfiguration. But this time he was taken into the clouds and into heaven. His Earthly body transfigured permanently.
Now we the Church (The Militant Church) are his earthly body. We the Church have been given authority to do what Jesus did on earth. And among these things is the authority to forgive sin and or to retain sin.