Ought ye not to know. — Literally, is it not to you to know? A construction characteristic of the chronicler. Abijah contrasts the moral position of his adversaries with his own, asserting (1) that their separate political existence is itself an act of rebellion against Jehovah; (2) that they have abolished the only legitimate form of worship, and established in its place an illegal cultus and priesthood; whereas (3) he and his people have maintained the orthodox ritual and ministry, and are therefore assured of the divine support.
By a covenant of salt. — As or after the manner of a covenant of salt, i.e., a firm and unalterable compact (see Numbers 18:19). According to ancient custom, salt was indispensable at formal meals for the ratification of friendship and alliance; and only a “salt treaty “was held to be secure. Salt therefore accompanied sacrifices, as being, in fact, so many renewals of the covenant between man and God. (Leviticus 2:13; Ezekiel 43:24; Leviticus 24:7 in the LXX.)
The antique phrase, “covenant of salt,” is otherwise important, as bearing on the authenticity of this speech.