A.T. Robertson Commentary 1 Corinthians 16:20

"All the brethren salute you. Salute one another with a holy kiss." — 1 Corinthians 16:20 (ASV)

With a holy kiss (εν φιληματ αγιω). In the synagogue men kissed men and women kissed women. This was the Christian custom at a later date and apparently so here. See 1 Thessalonians 5:26; 2 Corinthians 13:12; Romans 3:8; 1 Peter 5:14. It seems never to have been promiscuous between the sexes.