Diogenes' Dog said:
Religion is very old. It seems to have come naturally to us as humans at a very early stage in our history. The concepts, beliefs and values of a religion need to be taught, but the drive is already there (at least in some people).
Religion--or superstition, or belief in the supernatural--is an archetype, to use Jung's term. It occurs in all cultures and all eras. We have a hard-wired instinct to believe in something that can never be proven or disproven, in order to explain things that we don't understand or that bother us.
Like most instincts, it can be overridden by reason. Things our elders teach us or things that we experience on our way through life. But it can also be strengthened by those things.
Humanism can be seen as a codification of certain other, deeper instincts. Our instincts to improve productivity by working cooperatively and to improve health and safety by caring for each other increase the survivability of the species and motivated us to become a social species of pack animals. (As opposed to herd animals who gather in larger, anonymous, passively cooperative groups for peaceful grazing and defense against predation.) This occurred in our primate ancestors, several species differentiations back, and became much stronger as we evolved into the only predatory primates. (Other apes eat small animals casually caught but they subsist on vegetation, fruit, and bugs.)
Everything that humanism sets forth as reasonable rules for a harmonious civilization derive from our instinctive need to get along with each other so we can be healthy, productive, long-lived, and raise healthy, productive, long-lived offspring, while diverting as little energy, attention, and resources as possible to protecting ourselves from each other.
Religion claims to do this by relying on the supernatural and following often arbitrary rules whose relationship to reality is not obvious and often counterintuitive. As a result religions often veer away from the common-sense principles of peace, tolerance and cooperation.
The essence of humanism is that we are in touch with our nature, which leads us toward peace, tolerance and cooperation.
Humanism is natural, religion is supernatural. That is the fundamental difference. To call humanism a religion is to ascribe a supernatural element to it that it does not have, and must not have in order to work.