@NEETzsche Strictly speaking, it is about race mixing. The sin of both Israel and Judah were mixing with the peoples around them (which caused them to worship other gods, modern people will try to disconnect these two things, but they're always paired together in the Old Testament.)

Xenophon is a bit of a heretic though, to my understanding, he believes the New Testament is a racial covenant, which Paul consistently refutes. That doesn't mean race is irrelevant or doesn't exist, but it is no longer a barrier between Christian brothers, so a modern application of Ezekiel would be to avoid mixing with non-Christians.