And I would posit that the foremost modern audience for this exhortation is the church, the body of believers, they are believing, but they could believe a lot more than they already believe, they are shallow.
If this happens, salt and light will spill over into secular spheres and have a beneficial influence.
Sometimes I see we’ve tried to get Caesar to be our direct agents and that won’t work. The salt and light model works a lot better. Instead of asking teachers to bring God into schools, for instance... how about getting kids to do it. God really isn’t some quintessentially irascible old grump in the celestial regions, though a lot of grownups have styled Him in that fashion, more like themselves than like He really is. Kids who are on the groove with the character and power of Christ are so irrepressible with heaven’s optimism that a school full of them would not give up God even to an attack with a crow bar. And they themselves will be the witnesses.