The Greek adjective ethnikos refers to what belongs to the nations (ethnē) — the Gentiles or pagans as distinct from Jews. It carries the sense of one who lives according to Gentile/pagan customs, outside the covenant community of Israel.
Jesus uses ethnikos in Matthew 5:47 and 6:7 to contrast the behavior of disciples with that of pagans — even Gentiles greet their own kind and heap up empty words in prayer. The bar for kingdom living is set higher than cultural or ethnic in-group norms. Significantly, Paul's confrontation of Peter in Galatians 2:14 challenges him for 'living like a Gentile' — the new covenant has abolished the ethnic barrier, making all one in Christ.