16 Whence should the Jew have this kiss? For he who believed in His coming, believed not in His Passion. For how can he believe that He has suffered Whom he believes not to have come? The Pharisee, then, had no kiss except perchance that of the traitor Judas. But neither had Judas the kiss; and so when he wished to show to the Jews that kiss which he had promised as the sign of betrayal, the Lord said to him: “Judas, do you betray the Son of Man with a kiss?” that is, you, who have not the love marked by the kiss, offer a kiss. You offer a kiss who know not the mystery of the kiss. It is not the kiss of the lips which is sought for, but that of the heart and soul.
Source: Letters (New Advent)