I have been objecting to the calling of leftism a religion. Curiously, some people call atheism a religion. I object to that too.
The question as to what religion is is not at all easy to answer. It is not even clear that the question makes sense. For when you ask 'What is religion?' you may be presupposing that it has an essence that can be captured in a definition that specifies necessary and sufficient conditions. But it might be that the concept religion is a family resemblance concept like the concept game (to invoke Wittgenstein's famous example). Think of all the different sorts of games there are. Is there any property or set of properties that all games have and that only games have? Presumably not. The concept game is a family resemblance concept to which no essence corresponds. Noted philosophers of religion such as John Hick maintain the same with respect to the concept religion.
If you take this tack, then you can perhaps argue that Marxism and secular humanism and militant atheism are religions.
But it strikes me as decidedly odd to characterize a militant anti-religionist as having a religion. Indeed, it smacks of a cheap debating trick: "How can you criticize religion when you yourself have a religion?" The tactic is an instance of the 'So's Your Old Man' Fallacy, more formally known as the argumentum ad hominem tu quoque.
I prefer to think along the following lines.
Start with belief-system as your genus and then distinguish two coordinate species: belief-systems that are theoretical, though they may have practical applications, and belief-systems that are by their very nature oriented toward action. Call the latter ideologies. Accordingly, an ideology is a system of action-guiding beliefs. Then distinguish between religious and non-religious ideologies. Marxism and militant atheism are examples of non-religious ideologies while the Abrahamic religions and some of the Eastern religions are examples of religious ideologies.
I am using 'ideology' in a non-pejorative way. One could also speak of Weltanschauungen or worldviews except that 'view' suggests spectatorship whereas action-guiding belief-systems embody prescriptions and proscriptions and all manner of prudential dos and don'ts for participants in the flux and shove of the real order. We are not mere spectators of life's parade, but are 'condemned' to march in it too.
To repeat: there are theoretical belief-systems and belief-systems that are ineluctably action-guiding and purpose-positing. Among the latter we distinguish two subspecies, the religious and the non-religious.
But this leaves me with the problem of specifying what it is that distinguishes religious from non-religious ideologies. To put it Peripatetically, what is the specific difference? Perhaps this: all and only religions make reference to a transcendent reality, whether of a personal or impersonal nature, contact or community or identification with which is the summum bonum and the ultimate purpose of human existence. For the Abrahamic faiths, Yahweh, God, Allah is the transcendent reality. For Taoism, the Tao. For Hinduism, Brahman. For Buddhism, the transcendent state of nirvana. But I expect the Theravadins to object that nibbana is nothing positive and transcendent, being only the extinguishing or dissolution of the (ultimately illusory) self. I could of course simply deny that Theravada Buddhism is a religion, strictly speaking. I could lump it together with Stoicism as a sort of higher psychotherapy, a set of techniques for achieving equanimity, a therapeutic wisdom-path rather than a religion strictu dictu.
There are a number of tricky and unresolved issues here, but I see little point in calling militant atheism a religion, though I concede it is like a religion in some ways.
But as I have been pointing out lately, if one thing is like another, that is not to say that the one thing is the other or is a species of the other.
