Robert Green Ingersoll (/ˈɪŋɡərˌsɔːl, -ˌsɒl, -səl/; August 11, 1833 – July 21, 1899), nicknamed "the Great Agnostic", was an American lawyer, writer, and orator during the Golden Age of Free Thought, who campaigned in defense of agnosticism.

His father, John Ingersoll, was an abolitionist-sympathizing Congregationalist preacher, whose radical opinions caused him and his family to relocate frequently.

His father, John Ingersoll, was a Congregationalist minister and a man of mark in his time, a deep thinker, a logical and eloquent speaker, broad minded and generously tolerant of the views of others.

The popular impression which credits Ingersoll's infidelity in the main to his father's severe orthodoxy and the austere and gloomy surroundings in which his boyhood was spent is wholly wrong.

The Madison trial occurred when young Robert was nine years old, and it was the unjust and bigoted treatment his father received which made him the enemy, first of Calvinism, and later of Christianity in its other forms.During 1853, "Bob" Ingersoll taught a term of school in Metropolis, Illinois, where he let one of his students, the future Judge Angus M. L. McBane, do the "greater part of the teaching, while Latin and history occupied his own attention".

[3] Later that year, the family settled in Marion, Illinois, where Robert and his brother Ebon Clarke Ingersoll were admitted to the bar in 1854.

A county historian writing 22 years later noted that local residents considered the Ingersolls as a "very intellectual family; but, being Abolitionists, and the boys being deists, rendered obnoxious to our people in that respect.

[5] On November 11, 1856, Ingersoll caught Hall in his arms when the son of a political opponent assassinated his employer in their office.

For a time, Ingersoll represented con artist James Reavis, the "Baron of Arizona", pronouncing his Peralta Land Grant claim valid.

[10] With the beginning of the American Civil War, he raised the 11th Regiment Illinois Volunteer Cavalry of the Union Army and assumed command.

His opinions on slavery, woman's suffrage, and other issues of the time would sometimes become part of the mainstream, but his atheism/agnosticism effectively prevented him from ever pursuing or holding political offices higher than that of state attorney general.

Susan Jacoby credits Ingersoll for the revival of Thomas Paine's reputation in American intellectual history, which had decreased after the publication of The Age of Reason published during 1794–95.

Paine postulated that men, not God, had written the Bible, and Ingersoll included this work in his lectures on freethinking.

I see in Bob [Ingersoll] the noblest specimen – American-flavored – pure out of the soil, spreading, giving, demanding light.