Robert Green Ingersoll, agnostic writer and thinker (1833-1899):

Colonel Robert Green Ingersoll (August 11, 1833 – July 21, 1899) was a civil war veteran, American political leader, and orator during the Golden Age of Freethought, noted for his broad range of culture and his defense of agnosticism.

Recommended Reading:

Why i am a Agnostic

God in the Constitution