Matthew 5:16 (ESV) … “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.”
Jesus has just finished telling the disciples in verse 14, “You [emphatic: you alone] are the light of the world [and no one else!].” If we are true believers, we are the light of the world! To say such a thing about ourselves without divine precedent and sanction would be the height of arrogance. But Jesus said it, and it is easily one of the most amazing statements to ever fall from Christ’s lips—especially realizing what we are like when left to ourselves. It is a fact—we are light. 
However, we must remember that our light is a reflected or derived light. That light we shine forth does not originate from us. Ephesians 5:8 says, “for you were once darkness, but now you are light in the Lord. Live as children of light.”
Christ has worked a miracle in us by incorporating us into himself. This miracle on in Christ allows us to become that light, even however imperfect we may be as that light. The beauty of our being that light is that our light is derived entirely from Him—not a ray of it comes from ourselves—it is altogether reflective. Jesus allows us to “participate in His divine nature,” as
Peter says (2 Peter 1:4). This is a mystery.
This is a wonderful, beautiful mystery. Somehow as believers, we shine with the light of Christ, and it goes forth with life-changing effect. The facts here in Matthew 5 are: Jesus is the light; the world is in darkness; somehow believers are light.
How can we be sure to shine and shine brightly? We must stay close to Jesus! We must keep ourselves expose to Jesus. We must delight in His Word and spend time in prayer with Jesus soaking up His glorious rays.
 Hughes, R. K. (2001). The sermon on the mount: the message of the kingdom (p. 84). Wheaton, IL: Crossway Books.