Charles Ellicott Commentary Ezra 6:21

"And the children of Israel that were come again out of the captivity, and all such as had separated themselves unto them from the filthiness of the nations of the land, to seek Jehovah, the God of Israel, did eat," — Ezra 6:21 (ASV)

Separated themselves ... —Not proselytes from the Gentiles are intended, but the remnant of the Jews in the land who had associated with the foreign populations introduced by the conquerors. Their intermarriages and other acts of conformity are constantly referred to throughout Ezra and Nehemiah.