I think that when Jesus says this he is reminding his listeners of a promise made to Abraham the founding father of Israel that through him and his offspring ‘all nations will be blessed.’
I can’t think of another nation that was built for the purpose of not only securing peace and prosperity for it’s own people, but to be a blessing to the rest of the world.
How were the Jews supposed to be a blessing? Well, they illustrated that the Creator God loved His creation, that He could be found by people who seek Him, and by birthing the Messiah; Jesus.
God made it clear from the beginning that His people are there to bless other people. Jesus is doing the same in this verse, if His people are not fulfilling their purpose, if they are not being a blessing, then they are as useless as unsalty-salt.
Jesus said you are the salt of the earth. That means that God’s people need to be outward looking, our purpose isn’t to just look after one another, to enjoy the company of other Christians or pat ourselves smugly on the back. We exist to be a blessing to everyone else. Every good thing we receive we are meant to pass on for the benefit of others.
That is what I am going to be thinking about this week!