Religion
Humanism can be used in some ways to fulfill or supplement the role of religions in 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 blind 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.
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Knowledge
According to humanism, it is up to us to find the truth, not wait for it to be handed to us through revelation, mysticism, tradition, or anything else that is incompatible with the application of logic to the evidence. In demanding that we avoid blindly accepting unsupported beliefs, it supports scientific skepticism and the scientific method, rejecting authoritarianism and extreme skepticism, and rendering faith an unacceptable basis for action. Likewise, humanism asserts that knowledge of right and wrong is based on our best understanding of our individual and joint interests, rather than stemming from a transcendental or arbitrarily local source.