Humanism can be used in some ways to fulfill or supplement the role of religions in some people's lives, and therefore qualifies as a stance on religion. It is entirely compatible with naturalism (and therefore atheism), but doesn't strictly require either of these, and is compatible with some religions.
While the broad category of humanism encompasses intellectual currents running through a wide variety of philosophical or religious thought, it is embraced by some people as a complete lifestance. For more on this, see Humanism (lifestance).
Though the dominant forms of humanism are agnostic (and typically reject the existence of a supernatural), not all forms of humanism are. However, humanism denies the importance of the supernatural in human affairs, regardless of whether or not it exists. In this way, humanism does not necessarily rule out some form of theism or deism, and there are many humanists who consider themselves religious, some of whom are members of (typically, liberal) religious organizations. What humanism clearly rejects is deference to supernatural beliefs in resolving human affairs, not necessarily the beliefs themselves.
For that matter, agnosticism or atheism on its own doesn't necessarily entail humanism. Indeed, many different and incompatible philosophies are atheistic in nature.
Renaissance humanism, and its emphasis on returning to the sources, contributed to the Protestant reformation in helping to gain a more accurate translation of Biblical texts.
Sixth century B.C. pantheists Thales of Miletus and Xenophanes of Colophon prepared the way for later Greek humanist thought. Thales is credited with creating the maxim "Know thyself", and Xenophanes refused to recognize the gods of his time and reserved the divine for the principle of unity in the universe. Later Anaxagoras became the first freethinker and contributed to the development of science as a method of understanding the universe. Pericles, a pupil of Anaxagoras, influenced the development of democracy, freedom of thought, and the exposure of superstitions. Although little of their work survives Protagoras and Democritus both espoused agnosticism and a spiritual morality not based on the supernatural. The historian Thucydides is noted for his scientific and rational approach to history.
Renaissance humanism was a broad movement that affected the social, cultural, literary and political landscapes of Europe. Beginning in Florence in the last decades of the 14th century, renaissance humanism revived the study of the Latin and Greek languages; and caused the resultant revival of the studies of science, philosophy, art and poetry of classical antiquity.
The "revival", or "re-birth", was based upon interpretations of Roman and Greek texts, whose emphasis upon art and the senses marked a great change from the contemplation upon the Biblical values of humility, introspection, and meekness. Beauty was held to represent a deep inner virtue and value, and "an essential element in the path towards God".
The crisis of Renaissance humanism came with the trial of Galileo, which forced the choice between basing the authority of one's beliefs on one's observations, or upon religious teaching. The trial made the contradictions between humanism and traditional religion visibly apparent to all, and humanism was branded a "dangerous doctrine".
Renaissance humanists believed that the liberal arts (music, art, grammar, rhetoric, oratory, history, poetry, using classical texts, and the studies of all of the above) should be practiced by all levels of wealth. They also approved of self, human worth and individual dignity.
An important humanist from this period is the Dutch scholar Erasmus of Rotterdam.
One of the earliest forerunners of contemporary chartered humanist organizations was the Humanistic Religious Association formed in 1853 in London. This early group was democratically organized, with male and female members participating in the election of the leadership and promoted knowledge of the sciences, philosophy, and the arts.
In 1929 Charles Francis Potter founded the First Humanist Society of New York whose advisory board included Julian Huxley, John Dewey, Albert Einstein and Thomas Mann. Potter was a minister from the Unitarian tradition and in 1930 he and his wife, Clara Cook Potter, published Humanism: A New Religion. Throughout the 1930s Potter was well known advocate of women’s rights, access to birth control, "civil divorce laws", and an end to capital punishment.
Raymond B. Bragg, the associate editor of The New Humanist, sought to consolidate the input of L. M. Birkhead, Charles Francis Potter, and several members of the Western Unitarian Conference. Bragg asked Roy Wood Sellars to draft a document based on this information which resulted in the publication of the Humanist Manifesto in 1933. The Manifesto and Potter's book became the cornerstones of modern humanism. Both of these sources envision humanism as a religion.
In 1941 the American Humanist Association was organized. Noted members of The AHA include Isaac Asimov, who was the president before his death, and writer Kurt Vonnegut, who is the current honorary president.