John Lightfoot on Luke 14:34

[But if the salt have lost his savour.] This hath a very good connection with what went before. Our Saviour had before taught how necessary it was for him that would apply himself to Christ and his religion, to weigh and consider things beforehand, how great and difficult things he must undergo, lest when he hath begun in the undertaking he faint and go back; he apostatize, and become unsavoury salt.

Savour suits very well with the Hebrew word which both signifies unsavoury and a fool; Can that which is unsavoury be eaten without salt? Thy prophets have seen for thee vanity and that which is unsavoury. [Vain and foolish things, AV] The Greek, vain things and folly. He gave not that which is unsavoury to God. The Greek, he did not give folly to God, [nor charged God foolishly, AV].