Salute one another - Greet one another in an affectionate mannner; that is, treat each other with kindness and love, and evince all proper marks of affection.
With an holy kiss - This mode of salutation has been practiced at all times; and particularly in eastern nations. It was even practiced by “men;” see the note at Luke 22:47-48. The use of the word “holy” here serves to denote that Paul intended it as an expression of “Christian” affection; and to guard against all improper familiarity and scandal. It was common, according to Justin Martyr (Apology), for the early Christians to practice it in their religious assemblies.
The churches of Christ - That is, the churches in the vicinity of the place where the apostle wrote this Epistle; probably the churches particularly in Achaia.