15 And the same Scripture teaches you concerning the infusion of special grace, that he kisses Christ who receives the Spirit, where the holy prophet says: “I opened my mouth and drew in the Spirit.” He, then, kisses Christ who confesses Him: “For with the heart man believes unto righteousness, and with the mouth confession is made unto salvation.” He, again, kisses the feet of Christ who, when reading the Gospel, recognizes the acts of the Lord Jesus, and admires them with pious affection, and so piously he kisses, as it were, the footprints of the Lord Jesus as He walks. We kiss Christ, then, with the kiss of communion: “Let him that reads understand.”
Source: Letters (New Advent)