Genesis 48:5

As Reuben and Simeon, they shall be mine. — That is, Ephraim shall be regarded as my firstborn, and Manasseh as my second son. This was undoubtedly the case; for though “Judah prevailed above his brethren, and of him came the prince (and of him the Messiah), yet the birthright was Joseph’s” (1 Chronicles 5:2). The legal right of the firstborn was a double share of the father’s goods. This was bestowed upon Joseph in giving him two tribes, and to the other· sons but one. It was in a spiritual sense, and with reference to the promise that all mankind should be blessed in Jacob’s seed, that the birthright was Judah’s. As Joseph was the son of the chief and best-beloved wife, he had a sort of claim to the birthright; but in agreement with the law afterwards specially enacted (Deuteronomy 21:15), Jacob acknowledges that the right had belonged to Reuben, but excludes him from the possession of it as the penalty of his great and terrible sin. Simeon and Levi are next passed over, because of their cruelty, and so Judah takes Reuben’s place.