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 thy 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, thy sins are forgiven thee. While they were hastening to catch Him on the charge of blasphemy, they pledged themselves unawares to the truth. For Who is able to 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 ye do not believe in Me that I am God; yet abide ye 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.
Source: Homily - On Our Lord, 21