But all things that are reproved are made manifest by the light: for whatsoever doth make manifest is light.
Note 22 at Eph 5:13: It is a deception of the devil to think that our lives alone will shed God's light on the darkness of the world. It is true that speech without a life to back it up is hypocritical and often turns people away from the truth. But a godly lifestyle without openly expressing the source of our goodness only brings glory to ourselves. These verses instruct us to verbally express our witness.