The phrase all the brothers
probably refers specifically to the brothers and sisters in Ephesus, where Paul was staying at the time. It is as if the Corinthians are being embraced in the love of their Asian brothers and sisters. They should express this love towards each other with a holy kiss.
The adjective holy
probably does not describe a certain way of kissing. Rather, it describes the people who are to greet each other with a kiss. They are saints
(1 Corinthians 16:15), people who have been set apart for God.
This is not the only letter in which Paul commands believers to greet each other with a holy kiss
(see Romans 16:16; 2 Corinthians 13:12; 1 Thessalonians 5:26). Peter ends his first letter with a similar command, referring to this kiss as the kiss of love
(1 Peter 5:14).
In ancient times a kiss on the forehead or cheeks was a customary way of showing affection to one’s family members, friends, and guests (Luke 15:20). Jesus expressed some regret over the fact that, when he had entered the house of Simon the Pharisee, the latter had not greeted him with a kiss (Luke 7:45).
20 All the brothers send you greetings. Greet one another with a holy kiss.