Adam Clarke on Ezra 6:21

And all such as had separated themselves - These were the proselytes who had embraced the Jewish religion by having mingled with the Jews in their captivity. This proves that there the poor captives had so acted according to the principles of their religion, that the heathens saw it, and walked in the light of the Lord with them. A good example is very persuasive; and particularly so when founded on pure principles.