Let us start with the definition; a religion is "a group of beliefs". Humanism has beliefs, as many things do, and as such qualifies. It doesn't need any sort of pseudo-authority to say it is, as the only thing truly necessary is the idea that one can believe certain things, and if one can classify those into a group then something is a religion. As a humanist myself, I do find some claims ridiculous at best, to be blunt. Anyone willing to challenge me will be accepted, of course.
“Humanism is a progressive philosophy of life that, without supernaturalism, affirms our ability and responsibility to lead ethical lives of personal fulfillment that aspire to the greater good of humanity.”
– Humanist Manifesto III, 2003
The divine, in various forms, has played a central role in many cultures throughout human history. Human cultures across time have wondered how to find the divine, how to know the divine, and how to please divine. Humanists reject this focus. Placing human beings firmly in the center of their worldview, Humanists ask: “How, in this one life we have, might we make the most of our time here for ourselves and for others?” For Humanists, human concerns come first; they trump tradition, dogma, or creed. Humanists seek to discover what best promotes human flourishing while leaving behind those beliefs and practices that would prevent humanity from achieving its full potential.
This drive to improve human life can be expressed in three core values: reason, compassion, and hope. Humanists value reason, or the use of the intellect and practices like the sciences and philosophy, as the best way to generate accurate knowledge about the world we inhabit. They reject supernatural explanations for phenomena. They are driven by compassion, or the idea that all people—regardless of nationality, ethnicity, race, creed, sexual identity or other characteristic – are fundamentally of equal moral worth. Humanists also look to the future in hope, believing that human beings, if working together, can build a better world.
Many tributaries flow into the mainstream of Humanist thought, but two are particularly significant. Much of modern Humanism is inspired by the principles that animated the Enlightenment: a commitment to reason as a mechanism to change society and a commitment to science as the best way of learning about the world. The journals, salons, debating societies, and learned academies of the Enlightenment paved the way for the “marketplace of ideas,” a concept that characterizes modern culture and that Humanists embrace wholeheartedly. The staunch rationalism that pervades Humanism today is inspired by the spirit of these times. Humanists believe that people should be free to think and discuss any thought, regardless of the sacred truths that may be questioned by doing so.
The second major contributing influence comes from liberal religious movements, including liberal Christian and Jewish movements, Transcendentalism, and Unitarian Universalism. Over time such movements have tended to significantly deemphasize the role of God and the supernatural, moving closer to a position of outright Humanism. In fact, many of the signatories of the first Humanist Manifesto were religious liberals who found that their questioning of God’s role in the cosmos led them to Humanism. Humanistic Unitarian Universalist congregations continue to this day. It is largely from these religious traditions that the Humanist concern for the worth and dignity of all people is derived.
Religion is defined as "the belief in and worship of a superhuman controlling power, especially a personal God or gods." Humanism has no superhuman controlling power. It has now supernatural beliefs. If you're going to go off any group with a belief system is religious, then so are Capitalists, Communists, people who like James Bond and Vegetarians.
Humanism doesn't have a system (e.G. Any kind of rules like other religions), doesn't have a deity or anything of worship, doesn't have beliefs in the supernatural, or even a creation story. If you look at the definitions on Merriam Webster, Dictionary.Com, or the Oxford Dictionary, many of these are requirements for a religion, the closest Humanism gets is "a cause, principle, or system of beliefs held to with ardor and faith" in MW, although I wouldn't say Humanism is based on faith.