And that's...bad?
I wouldn't say it's bad as much as I would say that you are committing a logical fallacy, the appeal to authority. While it is, at times, appropriate to cite a legitimate authority as an expert of something, and while it is true that we can form very probably premises and truths from the thought of these figures, they are not necessarily always correct.
Because you didn't establish some
ethos, or, for that matter a qualifying point of view concerning Marx's expertise and genuine knowledge about religious things, your appeal to authority here is empty. You may as well quote Yogi Berra's famous "Baseball is ninety percent mental, and the other half is physical" as evidence for 50% = 10%.
When making arguments, never appeal to someone's thought simply because he is famous. Rather, establish the person's entire line of thought. Otherwise, you'll never get anywhere in an argument.