John 20:23Jesus gave this power to the Apostles, who were the first bishops. The office of bishop has been passed down since Matthias was chosen to fill the "office" of Judas."If you forgive the sins of any, they are forgiven them; if you withhold forgiveness from any, it is withheld.”
If the priest judges he is truly sorry, He must absolve since Christ's Passion merited forgiveness for every repentant sinner. Only if the person shows no willingness to give up sin does the priest retain, that is withhold absolution, as we "do not give what is holy to dogs" (Mt 7:6).
First, sacramental absolution from sin is not to be seen as something "instead of," or a "substitute for," forgiveness from God. Everyone who sins must, as soon as they are aware, repent of it and go to God in prayer for forgiveness. This is essential, and nothing can replace it.
The importance of sacramental Confession is that it is an outward sign, instituted by Christ to give grace.
An example of how God uses outward signs might be the historic OT incident where the Syrian general Naaman, on the advice of his wife's little child servant (!) goes to Elisha to be healed of his leprosy. God could have done it instantly, without Elisha. And Elisha--- by the power of God --- could have done it instantly as well, without any further action on Naaman's part. But instead, Elisha tells him he must dip himself seven times in the Jordan River.
At first, Naaman is contemptuous of this requirement, saying "Are not Abana and Pharpar, rivers of Damascus, better than all the waters of Israel?" But finally he accedes, trudges down to that inferior watercourse called the Jordan in the sight of all his fighting men, dips himself, and is freed of his leprosy.
What was the point?
The point is that God, through Naaman's wife's little Jewish servant girl, and through Elisha, had prescribed for Naaman the remedy he needed. He had to go, not to the King of Israel, but to this bushy-whiskered old coot in Samaria, and follow his words and immerse himself in this muddy runnel the Jordan, and adopt a little humility, a little docility toward the Lord and the man whom the Lord appointed. Only then would he be inwardly disposed to accept the miracle of healing in the right spirit.
Confession is like that. It inclines the soul toward truth (having to say out loud, audibly, just exactly what you did), toward docility before the Lord, who said to confess to other people (James 5:16), and toward full moral realism, recognizing that by sin we have offended and injured the whole Church, the Body of Christ of which we are members.
(I know, you said "concise" and here I am sermonizing on and on. But forgive me, I thought this needed some background. And though I am a female, I am a sermonizer, born to the breed! :o)
Anyhow, you asked:
"Must you go to a priest to ask for forgiveness[?]"
IN the case of mortal sin, yes. You must repent before the Lord right away through prayer and personal supplication, AND you must seek sacramental Confession at the very earliest opportunity. (This is not the case with venial sins, e.g. taking those paper clips from office supply.)
"...and can he withhold it?"
He could, but only when there is a manifest, objective reason to withhold.
"If he can withhold it, does that not make him equal to God in discerning a man's heart?"
No, because it can't be based on some presumed mystical insight into the penitent's heart. There must be a manifest (openly displayed) reason. Examples would be:
(I myself, personally, have never heard of a priest refusing absolution, but those are some of the situations where that would happen.)
The priest would have to make it clear to the penitent: "My brother, this is not a valid Confession,. Please come back as soon as you're sober." Or words to that effect.
P.S. to Arthur McGowan: is this pretty much correct?