Jesus said to them again, “Peace be with you. As the Father has sent me, even so I am sending you.”

And when he had said this, he breathed on them and said to them, “Receive the Holy Spirit. If you forgive the sins of any, they are forgiven them; if you withhold forgiveness from any, it is withheld.” John 20:21-23 (ESV)

Some argue that this verse proof that saints may also forgive sin against God. In addition, they may also refer to the following examples:

But Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back for yourself part of the proceeds of the land? While it remained unsold, did it not remain your own? And after it was sold, was it not at your disposal? Why is it that you have contrived this deed in your heart? You have not lied to man but to God.”

When Ananias heard these words, he fell down and breathed his last. And great fear came upon all who heard of it. The young men rose and wrapped him up and carried him out and buried him.

After an interval of about three hours his wife came in, not knowing what had happened.

And Peter said to her, “Tell me whether you sold the land for so much.” And she said, “Yes, for so much.” But Peter said to her, “How is it that you have agreed together to test the Spirit of the Lord? Behold, the feet of those who have buried your husband are at the door, and they will carry you out.”

Immediately she fell down at his feet and breathed her last. When the young men came in they found her dead, and they carried her out and buried her beside her husband. And great fear came upon the whole church and upon all who heard of these things. -- Acts 5:3-11 (ESV)

This was not offence that Peter took on himself. He judge Ananias and his wife based on the fact that they lied to the Holy Spirit of God. There was no way he could know this himself, except that the Spirit of the Lord revealed it to him. This passage has nothing to do with Peter's unwillingness to forgive their sin. It was a judgement that of God that Peter communicated.

When you are assembled in the name of the Lord Jesus and my spirit is present, with the power of our Lord Jesus, you are to deliver this man to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord. 1 Corinthians 5:4-5 (ESV)

Paul did not condemn the man to hell. Paul understood that sin has consequences even on Earth while we are alive. Paul judge the man to "destruction of the flesh so that his spirit may be saved". In other words, if he experiences enough pain or consequences, he may realize his error and repent so that his spirit may be saved from eternity. It is like a parent that punish his child with the purpose to save the child from a bad lifestyle.

By rejecting this, some have made shipwreck of their faith, among whom are Hymenaeus and Alexander, whom I have handed over to Satan that they may learn not to blaspheme. -- 1 Timothy 1:19-20 (ESV)

Paul gave the same judgement to Hymenaeus and Alexander so that they may "learn" their error.

Neither Peter, nor Paul are considered gods. Neither of them were acting from their own personal judgement or revenge. They were acting on behalf of "the Spirit" of God.

In none of these cases neither Peter, nor Paul decided themselves whose sin may or may not be forgiven against God.

Sometime prophets also acts as agents on behalf of God to declare God's punishment. For example:

Nathan said to David, "You are the man! Thus says the LORD, the God of Israel, I anointed you king over Israel, and I delivered you out of the hand of Saul. And I gave you your master's house and your master's wives into your arms and gave you the house of Israel and of Judah. And if this were too little, I would add to you as much more. Why have you despised the word of the LORD, to do what is evil in His sight?" -- 2 Samuel 12:7-9 (ESV)

It was not Nathan's personal choice or opinion to choose a punishment for David. Nathan was just the messenger delivering God's judgement to David.

Another example of a prophet that wanted judgement, but did not see it happen:

So Jonah arose and went to Nineveh, according to the word of the LORD. Now Nineveh was an exceedingly great city, three days' journey in breadth. Jonah began to go into the city, going a day's journey.

And he called out, “Yet forty days, and Nineveh shall be overthrown!”

And the people of Nineveh believed God. They called for a fast and put on sackcloth, from the greatest of them to the least of them.

...

When God saw what they did, how they turned from their evil way, God relented of the disaster that he had said he would do to them, and he did not do it.

But it displeased Jonah exceedingly, and he was angry.

And he prayed to the LORD and said, “O LORD, is not this what I said when I was yet in my country? That is why I made haste to flee to Tarshish; for I knew that you are a gracious God and merciful, slow to anger and abounding in steadfast love, and relenting from disaster.

-- Jonah 3:3-5; 3:10-4:2 (ESV)

Even great prophets like Jonah do not get to choose who is saved and who not.