The Sermon on the Mount, A Commentary, Part 2: Jesus' Instructions concerning How to Respond to His Law

Matthew 5:13-20 discusses  

13 * Ὑμεῖς ἐστε τὸ ἅλας τῆς γῆς·* ἐὰν δὲ τὸ ἅλας μωρανθῇ, ἐν τίνι ἁλισθήσεται; εἰς οὐδὲν ἰσχύει °ἔτι εἰ μὴ ⸂βληθὲν ἔξω⸃ καταπατεῖσθαι ὑπὸ τῶν ἀνθρώπων. 14 * Ὑμεῖς ἐστε τὸ φῶς τοῦ κόσμου.* οὐ δύναται πόλις κρυβῆναι ἐπάνω ὄρους κειμένη· 15 οὐδὲ καίουσιν λύχνον καὶ τιθέασιν αὐτὸν ὑπὸ τὸν μόδιον ἀλλʼ ἐπὶ τὴν λυχνίαν, καὶ λάμπει πᾶσιν τοῖς ἐν τῇ οἰκίᾳ. 16 οὕτως λαμψάτω τὸ φῶς ὑμῶν ἔμπροσθεν τῶν ἀνθρώπων,* ὅπως ἴδωσιν ὑμῶν τὰ καλὰ °ἔργα καὶ δοξάσωσιν τὸν πατέρα ὑμῶν τὸν ἐν τοῖς οὐρανοῖς. 

You are the salt of the earth. But if the salt becomes tasteless, how will it be made salty again? It no longer has any purpose except to be thrown outside and trampled underfoot by men. You are the light of the world. A city is not able to hide itself when it is seated high up on a hill. Nor do people light a lamp and hide it under a peck measure, but it gives light to everyone who is in the house. So also your light is to shine before men so that they might see your good actions and glorify your Father who is in the unseen realm.

The "you" is plural and so speaking to the disciples, and the assembly, as a collective. Given the context, however, He is likely talking to the disciples who represent church leadership (prophets, pastors, teachers). They are salt and light for the congregation, i.e., the household, by being examples of those who do good works that can be seen by those genuine Christians in their congregations.

Salt was something that was required with every offering given to God so that it would become pleasing to Him. Notice that it does not say that they taste like the salt of the earth but that they are the salt of the earth. The emphasis, again, is on being versus simply the action that takes place from being. Christ's new law stems from the new covenant, which is from root to fruit, based in a change of nature. Hence, if one were to become tasteless, evidencing that they are no longer the right kind of salt, the rhetorical question is asked to argue that there is no way for one to become salty again. This idea connects to the later ideas presented in Matthew such as the Parable of the Sower, where one who is initially declared to believe and be forgiven has that identification revoked due to evidence of his actions that expose a different nature. Salt is salty but unsalty salt isn't really the kind of salt that belongs to Christ and with which God is pleased. It no longer serves it purpose as something that is pleasing to God, and therefore, like so many other things analogous to being thrown out of the kingdom of God, it is thrown out to be tortured under the feet of men.

Likewise, those who truly follow Christ are lights, and as such, will produce light that can be seen by others. One might say he is a light but just a hidden light, but Christ rejects that here by saying that no real light is lit to be hidden. If one is truly a light, his good works will be seen by others. These works evidence the transforming work of God through the new covenant in his life that leads to actions noticeable by those within the "household," which likely refers to other Christians who are genuinely lights and not necessarily the world made up of pagans and false Christians that persecute the true Christian because of Christ in him rather than glorify the Father for them. The good works that evidence salvation cause other genuine disciples of Christ to glorify God for His marvelous work of transformation in other Christians. The Father is unseen as He is in the heavens, but He is seen through the character of Christians who are pleasing to Him and display a picture of Him in the good that they do.

These two elements of salt and light will contrast those whose righteousness loosens the law rather than expands it to its fullest expression in the character of Christ, a righteousness that is of the scribes and Pharisees that does not come from a change in nature but is merely external and therefore seeks to limit righteousness to the bare minimum because it does not rejoice in it but rather sees it as a burden. The reason why those who are transformed taste salty is because they are salt. The reason why they give off light is because they are light. Such is not the case for the false Christian whose righteousness is forced by cultural and religious influences but not from internal transformation.

17 * Μὴ νομίσητε ὅτι ἦλθον καταλῦσαι τὸν νόμον ἢ τοὺς προφήτας· οὐκ ἦλθον καταλῦσαι ἀλλὰ πληρῶσαι. 18 ἀμὴν γὰρ λέγω ὑμῖν· ἕως ἂν παρέλθῃ ὁ οὐρανὸς καὶ ἡ γῆ, ἰῶτα ἓν ἢ μία κεραία οὐ μὴ παρέλθῃ ἀπὸ τοῦ νόμου ⸆, ἕως °ἂν πάντα γένηται.* ⸇ 19 * ὃς ἐὰν οὖν λύσῃ μίαν τῶν ἐντολῶν τούτων τῶν ἐλαχίστων καὶ διδάξῃ οὕτως τοὺς ἀνθρώπους, ἐλάχιστος κληθήσεται ἐν τῇ βασιλείᾳ τῶν οὐρανῶν· ⸋ὃς δʼ ἂν ποιήσῃ καὶ διδάξῃ, οὗτος μέγας κληθήσεται ἐν τῇ βασιλείᾳ τῶν οὐρανῶν.⸌ 20 ⸋Λέγω γὰρ ὑμῖν ὅτι ἐὰν μὴ περισσεύσῃ ὑμῶν ἡ δικαιοσύνη πλεῖον τῶν γραμματέων καὶ Φαρισαίων,* οὐ μὴ εἰσέλθητε εἰς τὴν βασιλείαν τῶν οὐρανῶν

Do not think that I came to abolish the law or the prophets. I did not come to abolish but to bring them to their fullest expression. For truly I say to you, until heaven and earth pass away, a single yod or a single stroke of a letter will definitely not pass away from the law until everything comes to pass.

Christ makes it clear that the new covenant is not a contradiction to any of the righteousness declared by the entirety of the old covenant. Every bit of that righteousness, every bit of the character of God that is in the Hebrew Bible, will now come to its fullest expression in the follower of Jesus.

In light of this, anyone who starts fudging on any part of that righteousness, no matter how insignificant he may think it is, evidences that he is not a transformed person like the scribes and Pharisees that opposed Christ, will himself be considered insignificant by kingdom standards, and will therefore, not enter the kingdom of God.

This refers not only to someone who does not obey these things but anyone who teaches someone else that he or she can dismiss, ignore, go around, find loopholes and exceptions to obeying even the least moral command of God in the Hebrew Bible as it is now expressed in the fullness of the law of Jesus Christ is rejected by the kingdom as anyone belonging to it and will therefore not enter in. In this, we see that the Sermon on the Mount is not merely a warning to laity, although it is at points and with some application throughout but is instead primarily directed toward those who are doing the teaching, i.e., the teachers/prophets/pastors. As the scribes and Pharisees evidence that they do not know God by their lack of desire to fill up the moral commands of the Hebrew Bible to their fullest expression, so also these so called church leaders evidence that they do not know Christ, or rather are not known and transformed by Him, by virtue of their lack of fear and concern in minimizing the moral commandments of God that display His character to the rest of the congregation.