Albert Barnes on Romans 16:16

Salute one another. Greet one another in an affectionate manner; 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 practised at all times; and particularly in eastern nations. It was even practised by men. Lk 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 practise 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.

(y) "The churches of Christ" 1Cor 16:20, 1Pet 5:14