See the Matthew 5:13 note; Mark 9:49-50 notes.
Salt is good - It is useful. It is good to preserve life and health, and to keep from putrefaction.
His savour - Its saltness. It becomes tasteless or insipid.
Be seasoned - Be salted again.
Fit for the land - Rather, it is not fit "for land," that is, it will not bear fruit of itself. You cannot sow or plant on it.
Nor for the dunghill - It is not good for manure. It will not enrich the land,
Cast it out - They throw it away as useless.
He that hath ears - See Matthew 11:15. You are to understand that he that has not grace in his heart; who merely makes a profession of religion, and who sustains the same relation to true piety that this insipid and useless mass does to good salt, is useless in the church, and will be rejected. "Real" piety, true religion, is of vast value in the world. It keeps it pure, and saves it from corruption, as salt does meat; but a mere "profession" of religion is fit for nothing. It does no good. It is a mere encumbrance, and all such professors are fit only to be cast out and rejected. All such "must" be rejected by the Son of God, and cast into a world of wretchedness and despair. Compare Matthew 7:22-23; Matthew 8:12; Matthew 23:30; Matthew 25:30; Revelation 3:16; Job 8:13; Job 36:13.

Salt is good - See on Matthew 5:13 (note), and Mark 9:50 (note).
On the subject referred to this place from Luke 14:23, Compel them to come in, which has been adduced to favor religious persecution, I find the following sensible and just observations in Dr. Dodd's notes.
"1st. Persecution for conscience' sake, that is, inflicting penalty upon men merely for their religious principles or worship, is plainly founded on a supposition that one man has a right to judge for another in matters of religion, which is manifestly absurd, and has been fully proved to be so by many excellent writers of our Church.
"2nd. Persecution is most evidently inconsistent with that fundamental principle of morality, that we should do to others as we could reasonably wish they should do to us; a rule which carries its own demonstration with it, and was intended to take off that bias of self-love which would divert us from the straight line of equity, and render us partial judges betwixt our neighbors and ourselves. I would ask the advocate of wholesome severities, how he would relish his own arguments if turned upon himself? What if he were to go abroad into the world among Papists, if he be a Protestant; among Mohammedans if he be a Christian? Supposing he were to behave like an honest man, a good neighbor, a peaceable subject, avoiding every injury, and taking all opportunities to serve and oblige those about him; would he think that, merely because he refused to follow his neighbors to their altars or their mosques, he should be seized and imprisoned, his goods confiscated, his person condemned to tortures or death? Undoubtedly he would complain of this as a very great hardship, and soon see the absurdity and injustice of such a treatment when it fell upon him, and when such measure as he would mete to others was measured to him again.
"3rd. Persecution is absurd, as being by no means calculated to answer the end which its patrons profess to intend by it; namely, the glory of God, and the salvation of men. Now, if it does any good to men at all, it must be by making them truly religious; but religion is not a mere name or a ceremony. True religion imports an entire change of the heart, and it must be founded in the inward conviction of the mind, or it is impossible it should be, what yet it must be, a reasonable service. Let it only be considered what violence and persecution can do towards producing such an inward conviction. A man might as reasonably expect to bind an immaterial spirit with a cord, or to beat down a wall with an argument, as to convince the understanding by threats and tortures. Persecution is much more likely to make men hypocrites than sincere converts. They may perhaps, if they have not a firm and heroic courage, change their profession while they retain their sentiments; and, supposing them before to be unwarily in the wrong, they may learn to add falsehood and villany to error. How glorious a prize! especially when one considers at what an expense it is gained. But,
"4th. Persecution tends to produce much mischief and confusion in the world. It is mischievous to those on whom it falls; and in its consequences so mischievous to others, that one would wonder any wise princes should ever have admitted it into their dominions, or that they should not have immediately banished it thence; for, even where it succeeds so far as to produce a change in men's forms of worship, it generally makes them no more than hypocritical professors of what they do not believe, which must undoubtedly debauch their characters; so that, having been villains in one respect, it is very probable that they will be so in another, and, having brought deceit and falsehood into their religion, that they will easily bring it into their conversation and commerce. This will be the effect of persecution where it is yielded to; and where it is opposed (as it must often be by upright and conscientious men, who have the greater claim upon the protection and favor of government) the mischievous consequences of its fury will be more flagrant and shocking. Nay, perhaps, where there is no true religion, a native sense of honor in a generous mind may stimulate it to endure some hardships for the cause of truth. 'Obstinacy,' as one well observes, 'may rise as the understanding is oppressed, and continue its opposition for a while, merely to avenge the cause of its injured liberty.'
"Nay, 5th. The cause of truth itself must, humanly speaking, be not only obstructed, but destroyed, should persecuting principles universally prevail. For, even upon the supposition that in some countries it might tend to promote and establish the purity of the Gospel, yet it must surely be a great impediment to its progress. What wise heathen or Mohammedan prince would ever admit Christian preachers into his dominions, if he knew it was a principle of their religion that as soon as the majority of the people were converted by arguments, the rest, and himself with them, if he continued obstinate, must be proselyted or extirpated by fire and sword? If it be, as the advocates for persecution have generally supposed, a dictate of the law of nature to propagate the true religion by the sword; then certainly a Mohammedan or an idolater, with the same notions, supposing him to have truth on his side, must think himself obliged in conscience to arm his powers for the extirpation of Christianity; and thus a holy war must cover the face of the whole earth, in which nothing but a miracle could render Christians successful against so vast a disproportion in numbers. Now, it seems hard to believe that to be a truth which would naturally lead to the extirpation of truth in the world; or that a Divine religion should carry in its own bowels the principle of its own destruction.
"But, 6th. This point is clearly determined by the lip of truth itself; and persecution is so far from being encouraged by the Gospel, that it is most directly contrary to many of its precepts, and indeed to its whole genius. It is condemned by the example of Christ, who went about doing good; who came not to destroy men's lives, but to save them; who waived the exercise of his miraculous power against his enemies, even when they most unjustly and cruelly assaulted him, and never exerted it to the corporal punishment, even of those who had most justly deserved it. And his doctrine also, as well as his example, has taught us to be harmless as doves; to love our enemies; to do good to them that hate us; and pray for them that despitefully use and persecute us."
From all this we may learn that the Church which tolerates, encourages, and practises persecution, under the pretense of concern for the purity of the faith, and zeal for God's glory, is not the Church of Christ; and that no man can be of such a Church without endangering his salvation. Let it ever be the glory of the Protestant Church, and especially of the Church of England, that it discountenances and abhors all persecution on a religious account; and that it has diffused the same benign temper through that State with which it is associated.

(7) Salt [is] good: but if the salt have lost his savour, wherewith shall it be seasoned?
(7) The disciples of Christ must be wise, both for themselves and for others: otherwise they become the most foolish of all.

Salt is good,.... See Gill on Matthew 5:13, Mark 10:50.

Salt, &c.--(See on Matthew 5:13-16; and Mark 9:50).

Salt therefore is good. See Matthew 5:13 and Mark 9:50, for notes. The Christian is the salt of the earth; the savour is the spirit of self-sacrifice; if it is wanting his life is worthless.

Salt - Every Christian, but more eminently every minister. Matthew 5:13; Mark 9:50.