William Burkitt on Matthew 5:13

Our Saviour compares Christians in general, and his ministers in particular, unto salt, for a double reason.

First, Because it is the nature of salt to preserve things from corruption and putrefaction, and to render them savoury and pleasant. Thus are the ministers of the gospel to labour and endeavour, by the purity of their doctrine, to sweeten putrifying sinners, that they may become savoury to God and man; and may be kept from being fly-blown with errors and false doctrine.

Secondly, Because salt has a piercing power in it, which subdues the whole lump, and turns it into its own nature: such a piercing power is there in the ministry of the word, that it subdues the whole man to the obedience of itself.

As it Christ had said, "Ye are to be preachers and patterns to the world; ye are appointed by your pure doctrine, and good conversation, to purge the world from that corruption in which it lies: but if you lose either soundness of doctrine, or the savour of a good conversation, you will be wholly useless, as to these great ends, and must expect to be cast off by me, as unsavoury salt is cast to the dunghill."