Chapter 5:13-20 The Salt & Light

5:13 Ye are the salt of the earth: but if the salt have lost his savor, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men.
Ye are the salt of the earth - Our Lord shows here what the preachers of the Gospel, and what all who profess to follow Him, should be; the salt of the earth, to preserve the world from putrefaction.
But if the salt have lost his savor - A Christian, who has lost the life of Christ, and the witness of his Spirit, out of his soul, may be likened to this salt. He may have the sparks and glittering particles of true wisdom, but without its unction or comfort. Only that which is connected with the rock, the soul that is in union with Christ Jesus by the Holy Spirit, can preserve its savor, and be instrumental of good to others. It is thenceforth good for nothing, but to be cast out and to be trodden under foot. Salt is good for nothing, but to make things savory, and preserve from putrefacation; and when it has lost its savor, it is of no use, neither to men nor beasts, as some things are when corrupted; so ministers of the word, when they have dropped the savory doctrines of the Gospel, as their usefulness is gone.

5:14 Ye are the light of the world. A city that is set on an hill cannot be hid.

Christ never contemplated the production of secret Christians.

5:15 Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house.

Neither do men light a candle - Which may be read impersonally, "a candle is not lighted".

5:16 Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.

Not that they may glorify you, but that they may glorify your Father who is in heaven.

5:17 Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.

Think not that I am come to destroy the law - Do not imagine that I am come to violate the law καταλυσαι, from κατα, and λυω, I loose, violate, or dissolve - I am not come to make the law of none effect - to dissolve the connection which subsists between its several parts, or the obligation men are under to have their lives regulated by its moral precepts; nor am I come to dissolve the connecting reference it has to the good things promised. But I am come, πληρωσαι, to complete - to perfect its connection and reference, to accomplish every thing shadowed forth in the Mosaic ritual.

5:18 For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
No cross of a “t” and no dot of an “I” shall be taken from God’s law. Its requirements will always be the same; immutably fixed, and never to be abated by so little as “one jot or one tittle.”

5:19 Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
Whosoever therefore shall break one of these least 

commandments - Which are to be understood 

---not of the beatitudes in the preceding verses, for these were not delivered by Christ under the form of commandments; 

---nor of any of the peculiar commands of Christ under the Gospel dispensation; 

---but of the precepts of the law, and might be said to be broke, loosed, or dissolved, as the word here used signifies, when men acted contrary to them.

5:20 For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.

that except your righteousness exceed the righteousness of the Scribes and Pharisees, ye shall in no wise enter into the kingdom of heaven. - He mentions the Scribes, because they were the more learned part of the people, who were employed in writing out, and expounding the law; and the Pharisees, because they were the strictest sect among the Jews for outward religion and righteousness; and yet, it seems, their righteousness was very defective; it lay only in an external observance of the law; did not arise from a purified heart, or the principles of grace.