"Freethought" (one word) is the label most often embraced by atheists, agnostics, and secularists of the 19th and early 20th centuries. Free thinking (two words) could lead one to uncommon conclusions regarding any number of topics; freethinking (one word) came to mean the process by which one came to uncommon conclusions about religion.
Freethinkers held that (either probably or certainly) God does not exist; that gods, scriptures, and religions are best understood as products of human action; that religion and government should be held separate; and that moral values should be sought in human experience, not in holy books. Of course, this is essentially identical to the worldview held by 20th and 21st century secular humanists.
During the 19th and early 20th centuries, when west-central New York State played the role southern California does today, freethinkers tended to be involved in a wide variety of radical social reform efforts. Most of the prominent American anarchists were atheists, for example. Critics of religion were prominent in the abolition movement; they were especially visible in opposing the idea that slavery was ordained by God. Freethinkers were over-represented among so-called sex radicals, who championed everything from the abolition of marriage to birth control, then a most controversial notion. They were also prominent among proponents of woman's rights and woman suffrage; indeed, two of the three leaders of the 19th century woman suffrage movement, Elizabeth Cady Stanton and Matilda Joslyn Gage, were explicit freethinkers.
Freethinkers of this period had other associations that many today would find surprising. There was considerable overlap between the freethought and Spiritualist movements. That seems strange today, when atheists and secular humanists are often associated with groups skeptical of paranormal claims. But it should be recalled that in its earliest years, Spiritualism was regarded by many as a "scientific" exploration of the afterlife whose results overturned many of the teachings of traditional churches. Also, little of the mountainous evidence of misdirection and fraud among spirit mediums had yet come to light.
Of course, there was no shortage of freethinkers whose activism was principally concerned with challenging religous dogmas. Among them were orator Robert Green Ingersoll, journalist Obadiah Dogberry, organizer C. D. B. Mills, and educator Andrew Dickson White. Also noteworthy was the New York Freethinkers Association, an organization of national significance which held two important conventions in Watkins, now Watkins Glen. The first of these conventions, in 1878, was the site of events that would lead indirectly to the standard for obscenity upheld by U. S. courts until 1959.