Character is what you are in the dark.

Dwight Lyman "D. L." Moody (1837–1899) American evangelist and publisher

Sermon

Attributed by his son in William R. Moody, D. L. Moody, ch. 66 (1930), but quoted without citation before that (e.g., in Saint Andrew's Cross (Nov 1907), and The Outlook (6 Jun 1917)).