“You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot.
“You are the light of the world. A town built on a hill cannot be hidden. Neither do people light a lamp and put it under a bowl. Instead they put it on its stand, and it gives light to everyone in the house. In the same way, let your light shine before others, that they may see your good deeds and glorify your Father in heaven.”
Matthew 5:13-16

“You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot.”
Matthew 5:13

Salt was a sign of covenant faithfulness.

“For when I brought your ancestors out of Egypt and spoke to them, I did not just give them commands about burnt offerings and sacrifices, but I gave them this command: Obey me, and I will be your God and you will be my people. Walk in obedience to all I command you, that it may go well with you. But they did not listen or pay attention; instead, they followed the stubborn inclinations of their evil hearts. They went backward and not forward.”
Jeremiah 7:22-24

"Israel's whole story, commands, promises and all, was going to come true in [Jesus]. But, now that he was here, a way was opening up for Israel--and, through that, all the world - to make God's covenant a reality in their own selves, changing behavior not just by teaching but by a change of heart and mind itself.”
—N.T. Wright

Bottom Line

The church is called to be "good" different.

“Jesus says ‘you are the salt’, so be who you are. You are the covenant through whom God’s promises are coming into the world. How the Hebrew disciples lived their lives would demonstrate God’s covenant... If you’re not being the covenant, there is no covenant.”