By a covenant of salt? - For ever. "For as the waters of the sea never grow sweet, neither shall the dominion depart from the house of David." - Targum. See my note on Numbers 18:19; (note).
5. Gave the kingdom. Abijah upbraided the people of Israel for their revolt, asserting that they had no right to an independent existence apart from Judah, for God gave the kingdom to David forever.
Covenant of salt. A firm and inviolable compact (see on Num. 18:19).