the problem with leftism being similar to religious belief isn’t that defiant conviction is a bad thing, quite the contrary really, but that it tends to become a source of comfort and secure identification in the present rather than a drive towards its practical overcoming 
since this is a bit abstract, there are people who will get upset if you treat any obstacle to leftist goals, be it reactionary ideologies or the historical unfolding of capitalism, as anything other than a problem that is already sufficiently addressed by the left 
Two tweets from @adornofthagn, posted the other day, that I am thinking about a lot. Posted here because I will inevitably want to refer back to them again in future.
I have been writing about an inner and outer left in various places recently, following on from a recent reading of the “Outsideness” post on Xenosystems in which Nick Land comments on an understanding of an Inner- and Outer-NRx.
The left has no understanding of itself alone these lines but it could do with one — @adornofthagn nails it on why.