19 Wherefore, when the Pharisees, who seemed righteous to themselves, blamed the Lord, because, as physician, he mixed with the sick, and when they said, “Behold with whom he eats, with publicans and sinners,” the Physician replied to the madmen, “They that are whole need not a physician, but they that are sick: I came not to call the righteous, but sinners.” That is to say, because you call yourselves righteous when you are sinners, because you judge yourselves to be whole when you are languishing, you put away from you the medicine, and do not hold fast health.
Hence that Pharisee who had asked the Lord to dinner, was whole in his own eyes; but that sick woman rushed into the house to which she had not been invited, and, made impudent by the desire of health, approached not the head of the Lord, nor the hands, but the feet; washed them with tears, wiped them with her hair, kissed them, anointed them with ointment—made peace, sinner as she was, with the footprints of the Lord. The Pharisee who sat at meat there, as though whole himself, blamed the Physician, and said within himself, “This man, if he were a prophet, would have known what woman touched his feet.”
He suspected that He knew not, because He did not repulse her to prevent His being touched with unclean hands; but He did know, He permitted Himself to be touched, that the touch itself might heal. The Lord, seeing the heart of the Pharisee, put forth a parable: “There was a certain creditor, which had two debtors; the one owed five hundred denars, and the other fifty; and when they had nothing to pay, he frankly forgave them both. Which of them loved him most?” He answered, “I suppose, Lord, he to whom he forgave most.”
And turning to the woman, He said unto Simon, “Do you see this woman? I entered into your house, you gave me no water for my feet; but she has washed my feet with tears, and wiped them with the hairs of her head: you gave me no kiss; she has not ceased to kiss my feet: you gave me no oil; she has anointed my feet with ointment. Wherefore, I say unto you, to her are forgiven many sins, for she loved much; but to whom little is forgiven, the same loves little.” That is to say, you are more sick, but you think yourself whole; you think that little is forgiven you when you owe more.
Well did she, because guile was not in her, deserve medicine. What means, guile was not in her? She confessed her sins. This He also praises in Nathanael, that guile was not in him; for many Pharisees who abounded in sins said that they were righteous, and brought guile with them, which made it impossible for them to be healed.
Source: Tractates on the Gospel of John (New Advent)