Not a person but an organization, the relatively short-lived National Liberal League was the dominant national freethinkers' organization during the period known as the Golden Age of Freethought. It was founded in Philadelphia on the occasion of the national centennial, July 1 - 4, 1876. It adopted as its declaration the "Nine Demands of Liberalism," published three years earlier in the freethought newspaper The Index. The Nine Demands would define the secularizing agenda of American freethinkers for many years to come.
The league held its first annual meeting at Rochester's Corinthian Hall on October 26, 1877. Its second annual conference was held at Syracuse's Wieting Opera House on October 26 and 27, 1878. This closely followed the New York Freethinkers' Association's convention at Watkins Glen, where atheist publisher D. M. Bennett was arrested for selling a marriage-reform tract. Bennett's stature within the League, and its support for a radical sex-reform and birth-control agenda, led the agnostic orator Robert Green Ingersoll, who was somewhat socially conservative, to resign as vice-president of the League at its next annual meeting.
At its meeting in Cleveland in 1885, the League renamed itself the American Secular Union. Ingersoll rejoined as president. After various leadership changes the organization withered around 1919.
In 1945 another organization was founded under the name National Liberal League. Unrelated to the original, it remained a fringe group during its roughly fifteen-year lifespan.