John Chrysostom
Then, raising them up to love, the Apostle says, "Greet one another with a holy kiss" (v. 12). What does "holy" mean? Not feigned, not treacherous, as Judas kissed Christ.
The kiss was given to stir up love, to kindle sincere affection, so that we may love one another as brothers love brothers, as children love fathers, and fathers love children, or even more.
For that love is from nature, but ours is from grace. Through the kiss souls are joined together.
This is why, when returning from journeys, we kiss one another, when our souls enter into mutual communion. The kiss of the mouth especially expresses the disposition of our soul.
After saying, "Greet one another with a holy kiss," the Apostle added, "All the saints greet you," thereby giving them joyful hope.
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He wrote this in place of a kiss, joining them together by this mutual greeting. For from the same mouth come both the kiss and the words.
Do you see how he joins together all, those distant in body and those near? He joins the former by writing, and the latter by the kiss.
What does "holy" mean? Not feigned, not treacherous, as Judas kissed Christ.
The kiss was given to be a burning coal of love, to kindle affection, so that we may love one another as brothers love brothers, as children love fathers, and fathers children, or even much more.
For that love is from nature, but ours is from grace. Thus souls are joined to one another.
Therefore, when returning from journeys, we kiss one another, because our souls meet for mutual communion. The kiss of the mouth above all expresses the joy of our soul.
One may say something else about this holy kiss. What is it? We are the temple of Christ (cf. 1 Cor 3:16). Therefore, in kissing one another, we kiss the vestibules and entrance of this temple.