The Basics of Judaism: Jewish Texts: Ephraim as the First Born:
Why in Jeremiah 31:9 is Ephrayim referred to as G-d's first born?
Ephrayim wasn't even Joseph's first born; his brother Menasheh was.
But in Ch.31 Jeremiah is prophesying that the Northern kingdom (the
Kingdom of Israel) would return from exile and come back into G-d's
favor. Ephrayim is named because the tribe of Ephrayim played a
leading role in the Northern kingdom. He is called "first-born"
to indicate that "he is as beloved to G-d as a first-born son is
to his father" (Commentary Metzudas Tzion on v.9); see also v.19.
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