Because Joseph (v. 8) and Manasseh, one of Joseph's two sons (Gen. 48:5), represent two different tribes, Joseph will still have the double portion of the birthright (1 Chron. 5:1-2) during the millennium (Ezek. 48:4-5).
In the account here, as in 1 Chron. chs. 2—9, the tribe of Dan was omitted because of its idolatry (Judg. 18:30-31; 1 Kings 12:29-30; 2 Kings 10:29; cf. Gen. 49:17). However, Dan will still be counted during the millennium (Ezek. 48:1) because Jacob's blessing was upon him, so that through the Lord's salvation Dan will still be included as one of the tribes (Gen. 49:16-18).
Search