Secular Humanism and post-modernism are arguably related if not derived from each other. Which came first is a chicken-or-egg question.
All are forms of subjectivist Utopianism — which are rooted as far back as Plato. They all deny objective truth and values, believing instead that people can be made to conform to an arbitrary ideal of how they think things should be, which will work if enough people believe in it — as I explain in the OP.
*Marxism, Secular Humanism and post-modernism
It’s been a looonngg time since I studied Plato, but as I recall he believed in realities so objective we couldn’t even comprehend them directly.
I agree that secular humanism and post-modernism are essentially the same thing.
BTW, I always wondered what came after post-modernism. Post-post-modernism? The very name has always seemed to me to be a classic illustration of their self-absorption. Couldn’t even imagine that something might be around after they were gone.