Freethought (one word) is the label most often embraced by atheists, agnostics, and secularists of the nineteenth and early twentieth 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 unorthodox 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 twentieth and twenty-first–century secular humanists.
Writing in 1943, Columbia University historian and two-time Pulitzer Prize winner Allan Nevins described "the vigorous contest between freethinkers and religionists which was so important a part of the cultural history of the nineteenth century," and judged that "the battle which the freethinkers waged was in great part a wholesome and beneficial effort."
During the nineteenth and early twentieth centuries, when west-central New York State played a social role much like the one southern California played later in the twentieth century, 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 controversial notion. They were also prominent among proponents of woman’s rights and woman's suffrage; indeed, two of the three leaders of the nineteenth-century woman's suffrage movement (nineteenth-century practice was to use the singular, woman's; later practice was to use the plural, women's), 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 that is now available had yet come to light.
Of course, there was no shortage of freethinkers whose activism was principally concerned with challenging religious 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 that held its initial convention in Huron in 1877. This was followed by two important conventions in Watkins, now Watkins Glen. The first of the Watkins conventions, held in 1878, was the site of events that would lead indirectly to the standard for obscenity upheld by U.S. courts until 1959.