In Matthew 5:13-16 Jesus gives his followers some general, guiding principles on what they are to do in the world.
13 “You are the salt of the earth, but if salt has lost its taste, how shall its saltiness be restored? It is no longer good for anything except to be thrown out and trampled under people’s feet.
14 “You are the light of the world. A city set on a hill cannot be hidden. 15 Nor do people light a lamp and put it under a basket, but on a stand, and it gives light to all in the house. 16 In the same way, let your light shine before others, so that they may see your good works and give glory to your Father who is in heaven.
First we see the idea we discussed yesterday, Jesus tells his followers that they themselves are salt and light, not that there work will be salt and light or produce salt and light. Their saltiness and light-iness (?) is an outworking of who they are. This means they cannot seek to be salt and light without being the people Jesus called them to be (which Jesus will outline in the rest of the Sermon on the Mount). They cannot attempt to be salt by hating their brothers and sisters in Christ; they cannot be salt and light by lying, by bullying their enemies, etc. These indicate that Jesus followers are not the types of people Jesus called them to be.
Essentially the metaphor of salt implies that as disciples of Jesus we need to be useful to the world. We should be influencing their culture by preserving what is good, seasoning the lives of people around them to make them better, etc. Without influence and contact with the culture Christians cannot do this. So they must be engaged.
The light metaphor implies holiness and revelation. Christians are to represent Jesus well in their culture. They must shed light in dark spaces and illuminate the truth of God in their culture. While being engaged in the culture they must still remain holy within it.
So the images of salt and light form a twofold declaration on the Christian life. Followers of Jesus are to engage with the culture by being a useful, productive, positive presence in their communities. They are also to be distinct from the culture by seeking to dispel darkness, illumine truth, remove evil from wherever they find it, and represent the holiness of God.
whatever career path you find yourself in or being called to (whether sacred or secular), be salt and light and you will be fulfilling God's call in your life. Through this series I hope that you have felt a burden off your shoulders of finding God's specific call upon your life that you can't miss or you will be outside of his will. I also hope we have dispelled the illusion that you must be working for the church, doing spiritual work to be serving God. If you're just working a regular job you are fulfilling the creation mandate to make something of creation. You're also working with Jesus towards the redemption of all creation. Those are big things. Your work has intrinsic theological value. You can be salt and light in those settings. So, as Scripture says in Eccl. 9:10 and Col. 3:23, "whatever your hand finds to do, do it as unto the Lord".