John Gill Commentary 2 Chronicles 13:5

"Ought ye not to know that Jehovah, the God of Israel, gave the kingdom over Israel to David for ever, even to him and to his sons by a covenant of salt?" — 2 Chronicles 13:5 (ASV)

Ought you not to know
They did know what he afterwards says, but he would have them consider and acknowledge it:

that the Lord God of Israel gave the kingdom over Israel to David for
ever ;
to him and his seed, particularly to the Messiah, that should spring from him; but whether Abijah had this in view is a question, see (2 Samuel 7:13 2 Samuel 7:16)

even to him and to his sons by a covenant of salt ?
that is, a perpetual one, which was inviolable, and never to be made void; called so, because salt preserves from corruption and putrefaction, and because made use of in sacrifices offered when covenants were made; the Targum is, "as salt waters, which never lose their saltness."