It is true that they are mirror images of each other but neither believes in "the essential goodness of mankind" ("if men were angels" .. remember)
Both believe that historical necessity has a predetermined end, and that the end will be "good" -- social justice in one case, equilibrium in the other. This to me is a belief in Providence. And to be fair almost everybody believed in Providence in the 19th C., including agnostics. Or more precisely, virtually all belief systems were underlain with a belief in Providence.
As far as Marxism being a religion, some people will make a religion out of anything. Marxists themselves believe that their theories are science.
I admire Marx and Engels moralists, journalists, rhetorician, and social historian, but I don't buy their eschatology.
I also usually admire William Pfaff, but in waving about the spectre of "belief in the goodness of mankind" he is guilty sloganeering -- or coded language. Not sure what he is trying to imply, though. Maybe he is just on automatic pilot.
When you look closely at the usual suspects who are hauled before the bar and accused of "belief in the essential goodness of mankind", you find that they actually believed no such thing. It is a straw man argument.