Christianity has a strong humanist core in the teachings of Jesus Christ. Much the same is true of Judaism and Islam and their prophets. And much of the specifics of that core are unoriginal / inherited / shared around many other social and religious traditions – love for fellow man, the golden rule n’all that. Origin stories and creation myths too.
In order to emphasise our natural atheistic distinction versus supernatural theistic nature of (most of) these religions, most of us humanists reject any suggestions of being a religion. God forbid. It establishes clear water between organised humanism and organised religions where the latter depend on rules associated with teachings of their prophets posthumously recorded in their great books. Rules which their organised churches may enforce as dogma, even if great debates and schisms continue on interpretations, and many adherents may accommodate with the pragmatics of everyday living.
Humanism however has some key worldview aspects – values – we share as humanists, whether signed-up as bona-fide members of any formal humanist organisation – such as Humanists UK in my case – or not. In my book, those views which bind us in that shared identity do make us a religion by definition – religiare – that which binds us. And some of those values are pretty axiomatic if not dogmatic.
One of those is the natural view – the rejection of any supernatural deity – philosophy as natural science and the ecology of humanity within that. A lot more is said about that in this Psybertron project.
The other is the human aspect itself. Essentially since 1948 stemming from the Universal UN Declaration of Human Rights and subsequent freedoms of thought and expression and shared responsibilities for the global ecosystem. With acknowledgements due, of course, to all the precursor thinkers and campaigners that led to these being adopted. Axioms which are not beyond being legally enforced, by socio-economic political pressures and by force of physical intervention. Axioms many of which are also enshrined and protected in national legal systems. Axiomatic by means of constitutional and revisable democratic arrangements, but not so democratic we wouldn’t all be outraged if a populist movement ignored or overturned them?
It is only ignorance of the naturalistic fallacy that prevents most humanists accepting that these humanist axioms are not themselves natural, and depend on being maintained by collective human will. Humanism is the most widespread religion in all but accepted identity.