21 Now the Pharisee, while he was doubting concerning our Lord, that He was not a prophet, pledged himself to the truth unawares, in saying— Had this man been a prophet, he would have known that this woman is a sinner. Therefore, if it should be found that our Lord knew that she was a sinner, He is, according to your word, O Pharisee, a prophet. Our Lord, therefore, hastened to show both that she was a sinner, and that her sins were many; that the testimony of his own mouth might confute him as a liar. For he was companion of those that said: Who is able to forgive sins, but God only? For from them our Lord received testimony, that, therefore, He Who is able to forgive sins, is God. Thenceforth, then, the contention was this, that our Lord should show them whether He was able to forgive sins or no. So He speedily healed the members that were visible, that it might be made sure that He had forgiven the sins that were invisible. For our Lord cast before them the word which was expected to catch him that said it; so that when they should rush forward to catch Him by it, according to their wish, they might be caught by Him according to His wish. Fear not, My son, your sins are forgiven you. While they were hastening to catch Him on the charge of blasphemy, they pledged themselves unawares to the truth. For Who is able forgive sins but God only? Accordingly, our Lord confuted them [as though saying]: “If I shall have shown that I am able to forgive sins, even though you do not believe in Me that I am God; yet abide by your word, which determined that whoso forgives sins is God.” Therefore that our Lord might teach them that He forgives sins, He forgave that man his hidden sin, and caused him to carry his bed openly; that by the carrying of the bed which carries [those that lie on it], they might believe in the slaying of the sin that slays.
This is a wonderful thing, that while our Lord there called Himself the Son of man, His adversaries, unawares, made Him to be God as forgiving sins. Accordingly, while they supposed that they had ensnared Him by their craftiness, He entangled them in their craftiness; He made it a testimony to His truth. So their evil thoughts became unto them as bitter bonds; and that they might not free themselves from their bonds, our Lord strengthened them by giving strength to him [to whom He said]— Arise, take up your bed and go into your house. For the testimony could not again be undone, as though He were not God; inasmuch as He forgave sins. Nor yet could it be falsely affirmed that He had not forgiven sins; for lo! He had healed [men's] limbs. For our Lord bound up His hidden testimonies in those which were manifest; that their own testimony might choke the infidels. Accordingly our Lord made their thoughts to war against them, because they had warred with the Good One, who by His healing power warred against their diseases. For that which Simon the Pharisee imagined, and that which the scribes his companions imagined, they imagined in their hearts secretly; but our Lord spread it forth openly. Our Lord represented their hidden imaginations before them, that they might learn that His knowledge reveals and shows their secret things(;) so that though they had not recognized Him by His open signs, they might recognize Him when He represented their secret imaginations; and that if only but by this—that He searched out their hearts—their hearts might perceive that He was God—that at least when they saw that their imaginations could not be hidden from Him, they might cease from imagining evil against Him. For they had imagined evil in their heart; but He exposed it openly, by this [word] Why are you imagining evil in your heart? So that by this, that our Lord perceived their hidden imagination, they should recognize His hidden Godhead. For that Godhead, by this very thing that they in their error were reviling it, was by that reviling made known to them. For they reviled our Lord in the body, and supposed that He was not God, and cast Him down below from on high; but by the body He was made known to them as being God, by that body which was found passing to and fro among them. For they, by casting Him down to the depth, attempted to show this, that God Who is above, cannot in bodily wise be born below. But He by His passage up to the height, taught them this; that for the body also that is sent down below, it is not its nature to pass up to the height rather than down to the depths; so that by the body which from below passed on high upwards in the air, they might learn of God that by His grace He descended down below from on high.
Source: On Our Lord (New Advent)