Karl Marx, the secular Jesus.
With regards to a religion, yes, Marxism can be similar, but there are very certain, definite, chasm-like differences that you can't reconcile when you try to bucket Marxism as a religion.
Firstly, and this is a tangent but a useful one, the religion of Christianity has very little to do with Jesus - Jesus of Nazareth, the person who (likely) really lived and fought the Jewish aristocracy and Roman occupation. We know very little of that Jesus.
What people
believewas almost entirely made up by the Apostle Paul, who never knew Jesus and was at odds with his real followers, the ones that did know him. The majority of the NT is made up of Paul's writings, and he made lots of stuff up - lots of things that contradicted what Jesus supposedly said (compare The Epistle of James, which is night and day different than what's in the Bible). He's the one that gave him the Greek "Christ" title.
So Christianity grew from one man, but what most believe has nothing to do with that man. It has to do with other men writing about him, and in most cases, getting what he said, believed, and fought for wrong.
It's the same with Marxism. After Marx, lots of people interpreted what they thought was right, wrote even more literature in his name, and committed widespread atrocity and used his texts as an excuse. In this respect, Jesus and Marx are very similar: their words are taken and contorted to suit the ends of people wanting power, whether it was Constantine or Stalin.
The difference is though, with Marx, we can look to what he actually wrote, what he actually said. That's always been my modus operandi. I'll use a few Marxian authors here and there, such as Fromm and Gramsci, but you have to be very careful and consider their words independently of Marx's.
It would be a religion if I really didn't know what Marx said, attributed words written by people like Fromm and Gramsci to him, in his name, and I would be doing that by some screwed up idea of faith like the modern Christian version. I don't: I use Marx's words directly because he was a damned good scholar.