Numbers 18:19

A covenant of salt. It is a proverbial expression, signifying a covenant not to be altered or corrupted; as salt is used to keep things from corruption; a covenant perpetual, like that by which it was appointed that salt should be used in every sacrifice, Leviticus ii. 3. (Challoner) --- Thus God gave the kingdom to David for ever, by a covenant of salt, 2 Paralipomenon xiii. 5. Salt is an emblem of eternity. Oleaster believes, that salt was used in the ratification of all solemn covenants, to denote their stability. (Calmet)