Religion is a lot broader than Christian fundamentalism and zealots. It's sort of like applied philosophy: how do you live a flourishing life in relationship to other people and to the god(s). Modernity has an implicit materialist worldview (matter is all that is) and an explicit rejection of the divine. However, if matter is all there is, then there is no meaning in the world. This is not a way to flourish in the world. (And if we cannot flourish with materialist consequences, that is some evidence that the materialist assumption is incorrect.) So religion is not just some silly, backwater thing, and Marx was absolutely wrong.

The Christian fundamentalism you decry is the shriveled remains of a branch of Christianity that failed to protect itself from drying out in the heat of modernity. Fundamentalism is actually a reaction against modernity, but the East/West split cut off part of the philosophical richness, and the Protestant reformation cut off most of the rest of the philosophical richness, as well as the pathway to the mystical/transcendent. The Fundamentalists couldn't separate the indisputable truths of materialist analysis (Science) from the assumptions necessary for that analysis (materialism), and so they just rejected both. (Except, not really; they live as functional materialists with an exception for God.)

>However, if matter is all there is, then there is no meaning in the world. This is not a way to flourish in the world.

Things like this really make it hard, as an atheist, to receive the argument that my problem is with Christianity, and not with religion.

You're saying that my beliefs mean there's no meaning, and are incompatible with flourishing in the world. I understand you feel the need to defend your beliefs as valuable and important, but somehow it seems almost impossible for religious people to do so without denigrating atheism.

And yes, a lot of atheists are dismissive of religion too. But look, I'll show you: I personally don't find religion necessary to live an ethical and fulfilling life, but I understand that many people find it valuable and compelling, and that's ok as long as they let other people live their lives too. I think people can be intelligent, rational, and respectful of the beliefs of others, while still maintaining their own religious beliefs.

There, that wasn't so hard, was it?