We'll, one side is clearly confused....
If a preacher, at the end of an evangelistic sermon, says to his congregation (assuming there are unsaved people present), "Unless you repent of all your sins you cannot be saved," he is wrong.
If a preacher at a business meeting is following Jesus's guidelines set forth in Matthew 18, and is dealing with a member who has been immoral or unfaithful to his wife, and says, "unless you repent of your sin, you will not be welcome here," he is right.
John the Baptist was preaching to the Jews. He preached a message of repentance. He not only preached that they had to repent, but they had to bring fruit of their repentance before he would baptize them. But he was speaking to the Pharisees. At that time they were still God's chosen people.
The word metanaoi means to "have a change of mind."
It does not mean to forsake all your sins. That is reformation not regeneration.
He must have a change of mind with respect to his attitude toward God.
As an unsaved person his attitude is rebellious and sinful. By putting his faith in the Lord Jesus Christ, he is changing his attitude (a change of mind toward God). He is not rebellious now, but submissive to the will of God. That is repentance--a change of mind toward God. It is always a change of mind. That is what the word means.