6:21a The Israelites who had returned from exile ate it,

The people of Israel who had returned to Judah ate ⌊the Passover meal⌋.
-or-
Then the Jews who had come back home after living as foreigners ⌊in Babylon⌋ ate ⌊the sheep/feast⌋.

6:21b together with all who had separated themselves from the uncleanness of the peoples of the land

⌊They ate it⌋ together with all ⌊the local people⌋ who had turned away from the evil deeds of their pagan neighbors. These local people joined them
-or-
Other people ⌊who lived in that place⌋ also ate the feast meal with them. By doing so they showed that they rejected/left their bad ways ⌊of worshiping other gods⌋.

6:21c to seek the LORD, the God of Israel.

as worshipers of Yahweh, the God of Israel.
-or-
Now, like the Jews, they would pray to Yahweh, the God that ⌊the people of⌋ Israel ⌊pray to⌋.

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