Leftism is not a religion, it is an anti-religion; Leftism - like all religions, like all world views of any kind - is indeed based upon metaphysical assumptions.
The slur of accusing Leftism of being 'a religion' is that being 'a religion' is A Bad Thing. The implicit (if unarticulated) contrast is with the accuser's own world view - which is covertly assumed thereby Not to be 'a religion' but to be... what? 'Based on 'evidence', or 'observation', or 'facts' or something....
What the accuser is implying here is that a world view based upon basic assumptions is a religion - and that a religion is A Bad Thing because it is possible (and better) to have a world view that is Not based on assumptions, but based on evidence/ observation/ facts or whatever.
This is the incompetence of the slur. Because all world views are actually and always based on metaphysical assumptions.
It is these assumptions that define and validate whatever counts as evidence/ observation/ facts. Because - for the accuser of another world view 'being a religion' - some things count as evidence/ observation/ facts and other things do not.
But how to tell evidence/ observation/ facts from the other things that are not really evidence/ observation/ facts?
The usual (incompetent) answer is that these particular evidence/ observation/ facts under discussion) are validated by another bunch of evidence/ observation/ facts... OK, but what then validates those evidence/ observation/ facts?
In the end - if we are honest and competent, it is either an infinite regress which must be 1. false - or, 2. explains nothing - or else 3. 'infinite regress' is itself the metaphysical assumption!
Either one of them; or we get down to some metaphysical assumptions that are regarded as Just True.
But few people are competent thinkers, and even fewer are honest thinkers (the two are related, since competence follows honesty) - so people do not acknowledge the necessity of assumptions; and we get the accusation that "X 'is a religion' - whereas I personal am Not religious".
Therefore Leftism is Not a religion, but it is - of course - inevitably, based-upon metaphysical assumptions... But then everything is based on metaphysical assumptions - so saying 'Leftism is a religion' is either untrue, or thoughtless-meaninglessness parading as meaning; or else (too often) a dishonest attempt at propaganda.
The lesson? we all of us, every one, actually has a world view based on metaphysical assumptions that are unsupported by evidence/ observation/ facts.
The distinction ought-to-be between those who:
1. Acknowledge that they have metaphysical assumptions, and those who (ignorantly, incompetently or dishonestly) deny that they have metaphysical assumptions.
2. Those who know their metaphysical assumptions, and those who - while they acknowledge their existence - do not know them.
For me (and this is one of my metaphysical assumptions) - our destiny (in The West, among adults who are psychologically mature enough to read this) is to first acknowledge, then become-aware-of, our own metaphysical assumptions.
By become-aware-of I do Not mean 'communicate to other people', nor do I mean even 'articulate to ourselves' - what I do mean is to become aware of, apprehend, grasp intuitively, our own metaphysical assumptions.
If so, then...
3. We need to reflect deeply upon our own actual metaphysical assumptions; and discern whether these metaphysical assumption Which We Personally Have are valid.