Ezra 6:21
And the children of Israel, which were come again out of captivity, and all such as had separated themselves unto them from the filthiness of the heathen of the land, to seek the Lord God of Israel, did eat,
Cross-reference
Ezra 9:11 describes the land's filthiness from pagan practices — here the people separate themselves from that same filthiness to seek God.
Exodus 12:47-49 includes foreigners in Passover after circumcision—the legal precedent for Gentiles joining Israel's Passover as seen in Ezra.
Numbers 9:10-14 provides the law allowing the unclean to keep Passover later—the legal basis for inclusion that Ezra's group exemplifies.
Isaiah 52:11 commands 'touch no unclean thing; come out from it'—the same call to separation that characterizes those who joined Ezra's Passover.
2 Corinthians 6:17 quotes Isaiah 52:11, applying the separation principle to the NT church—echoing Ezra's call to separate from uncleanness.
2 Corinthians 7:1 urges purification from all defilement—a NT parallel to the voluntary separation practiced by those in Ezra 6:21.
Numbers 9:6 addresses defilement preventing Passover — here the people have purified and separated, enabling them to partake.