Jesus says, in Matthew 5:14, "You are the light of the world." And in Ephesians 5:8, "At one time you were darkness, but now you are light in the Lord." So both of them say, Christians are light. Not only that, they both agree that, "The fruit of light is found in all that is good and right and true."
The shining of your light as a Christian is bearing practical fruit in "good deeds" — "what is good and right and true." But Paul stresses a function of light that Jesus does not mention in Matthew 5, namely, that light exposes the dark. When light gets near a dark thing truth happens. Things are seen for what they are. The deceptions and half-truths are blown away.
Abortion is one of the darkest works of the human race — it is child sacrifice. And the only way it can survive is for darkness to survive. Wherever the light of truth and love comes, darkness flies away. Therefore it is one of the great callings of the followers of Jesus to let their light shine in both ways: to do good deeds and to expose darkness. The aim is partly negative: reveal the error hidden in the darkness, but mainly positive: to bring people to love the light and be made light in the Lord Jesus.
Excerpted from The Darkness of Abortion and the Light of Truth (2003).
Get the free e-book (PDF) from John Piper, Exposing the Dark Work of Abortion.