Light is a frequently used metaphor in scripture, and Jesus calls his disciples the light of he world. That is our identity, and in this context Jesus ties it to an expression of our identity as Christians by doing good works. In our witnessing, living as aliens, as light and doing good works will undergird and give legitimacy to the gospel we preach. Not for our own glory, Jesus denounces this in Matt 7, but humbly, for the glory of God our good works should shine before men.