18 Let us now see the rest concerning this man. “Behold an Israelite indeed, in whom is no guile.” What is “in whom is no guile?” Perhaps he had no sin? Perhaps he was not sick? Perhaps he did not need a physician? God forbid. No one is born here in such fashion as not to need that Physician. What, then, is the meaning of the words, “in whom is no guile”? Let us search a little more intently— it will appear presently— in the name of the Lord. The Lord says dolus [guile]; and every one who understands Latin knows that dolus is when one thing is done and another feigned.
Give heed, beloved. Dolus (guile) is not dolor (pain). I say this because many brethren, not well skilled in Latin, so speak as to say, Dolus torments him, using it for dolor. Dolus is fraud, it is deceit. When a man conceals one thing in his heart, and speaks another, it is guile, and he has, as it were, two hearts; he has, as it were, one recess of his heart where he sees the truth, and another recess where he conceives falsehood.
And that you may know that this is guile, it is said in the Psalms, “Lips of guile.” What are “lips of guile”? It follows, “In a heart and in a heart have they spoken evil.” What is “in a heart and in a heart,” unless in a double heart? If, then, guile was not in Nathanael, the Physician judged him to be curable, not whole. A whole man is one thing, a curable another, an incurable a third: he who is sick, but not hopelessly sick, is called curable; he who is sick hopelessly, incurable; but he who is already whole does not need a physician.
The Physician, then, who had come to cure, saw that he was curable, because there was no guile in him. How was guile not in him, if he is a sinner? He confesses that he is a sinner. For if he is a sinner, and says that he is a just man, there is guile in his mouth. Therefore in Nathanael He praised the confession of sin, He did not judge that he was not a sinner.
Source: Tractates on the Gospel of John (New Advent)