Today the health conscious urge us to use salt in moderation or not at all, and gourmets tell us to buy saltier (& more expensive!) salt. There's no pleasing everyone. But Jesus urges us to be salt. Scholars offer many explanations. None has any advantage over keeping it as simple as Jesus; taking him at face value. Either we live out / bring out the flavour of the Gospel, or we're useless! There's no in between in this case. An interesting suggestion is that some synagogues may have made apostates wanting to be received back into the fold lie down across the doorway of the meeting place, & that some early churches may have taken the idea on board. Salt that's lost its flavour is only for trampling on. What fun! Could we get 'A Form for the Stomping of Penitents' into the next Prayer Book? If we practised colourful mimes like this again, imagine the queues to stomp or be stomped! Wouldn't-be penitents might make themselves scarce as clergy on the bishop's defrocking party list! Enough of such frivolity. Seriously, how are we to prove we are salt / worth our salt, & always have been, or again?

We're used to light at the flick of a switch; in those days dark was dark & light was light & never the twain would meet till someone lit a lamp or a torch. That point lies behind all the darkness-light imagery of the Gospel. There's no mistaking Christians who're light from those who are not, i.e. are darkness, than there is salt from not-salt.
Jesus is challenging half-heartedness for what it is: in the first case, a failure to bring out the flavour of God in life, & now, secondly, a failure to reveal God's Glory.

Those who see keeping the Faith as a set of rules to be obeyed (or else!) often take refuge in this next bit. At first glance Jesus appears to be taking a hard-line approach, but think it through. If the Law is the expression of the Will of the God of Love, then its fulfilment, right down to the last jot & tittle, must also be an expression of that Love. Human nature finds it easier to lay down the Law (for someone else!) than to Love them. Easier to lay down Law than Love.
Jesus' summary of the Law reveals a fuller, more mature grasp of God's nature than anyone before him. After all, he does have inside information! God in Jesus is not some cantankerous being who switches between wrath & love depending upon how well we're keeping the rules.

Someone once said that God's wrath is only ever God's love seen from the other side. It's often   necessary to wean people away from the God of wrath it suits them to believe in (often from what they believe the Hebrew Bible & a selective reading of the New Testament teaches) & instead help them suckle on God's love. A lot of us are still 'Scribes & Pharisees' at heart, who haven't made the transition from Mt.Sinai to the Mt of the Sermon. Not to a different God; just a different way of understanding & enjoying God. That's the way our righteousness will exceed that of the gents afore -mentioned. The way we enter into & under God's Rule. If you still doubt God's wrath is the flip- side of God's love, flip the coin again. And keep flipping it till it comes up heads!