Matthew 9:6

“But that you may know that the Son of man has authority on earth to forgive sins (then he says to the sick of the palsy), “Arise, and take up your bed, and go up to your house.”

Having stunned them to silence Jesus then positively declared His position and His intention. In order that they might know that He truly was the Son of Man, the God-anointed One of Daniel 7, and therefore had the right while on earth to forgive sins He would perform a miracle. He would do what they could not do, what only One Who was approved of God could do. He would enable this man to walk. Then if they were honest, having failed to argue against His reasoning, they would have to admit His right to forgive sins.

So turning to the paralytic He told him to rise from his mattress and walk home carrying his mattress. What better proof could there be that he was genuinely healed, and therefore now coming under the approval of God, and therefore also forgiven.