For all the time I've spent looking in to it, I haven't seen a really comprehensive Marxist attempt to spell out why they are Atheist at all, beyond simply asserting they are materialist and it's "scientific". So personally, it remains an open question. It is a matter of dispute amongst Marxists, but for the sake of a quiet life and not alienating people I'd say it is ok and being a Christian and a Marxist isn't really going to bother anyone.
Marxism is roughly composed to two sets of ideas; Dialectical Materialism, which is a philosophy of science and nature, and Historical Materialism, which is a theory of science and history.
Most people have some familiarity with
Historical Materialism and will borrow Marxist ideas and use them in combination with religious ideas, including Christianity. If you go to a sociology class at university or school, you'll learn basic ideas about "class", "base and superstructure", "capitalism", "socialism", etc. If you go to a bookstore and pick up a book on Marx and Marxism, this is typically what you will read about and authors will give various perspectives on it.
These are really the building blocks of Marxist ideology and can be used any number of ways depending on what you are trying to do. Everyone will probably read or have heard of the Communist Manifesto at some point, and these ideas have trickled in to the wider public consciousness over time. Often this means we adopt some Marxist ideas whilst unwittingly holding existing ideas which may not be Marxist or are even opposed to Marxism. In the long-run, you will probably run in to philosophical problems but you can deal with that if and when they arise.
However,
Dialectical Materialism is most
definitely an atheist philosophy as it is essentially a theory of knowledge which determines how you can know something, what is true and what is possible when you look at natural and social phenomenon. It takes a fairly explicitly Atheist position rejecting the existence of god, the soul, supernatural phenomena in general and asserts that there could never be a "creation" of the universe (or matter). It would assert that religion is a "false consciousness" of nature and society that preserves an exploiting society in contrast to the "scientific socialism" of Marxism. This is the philosophical thinking behind Marxism-Leninism that defined the Soviet Union and other Communist bloc countries, hence why they were "State Atheist" and "Anti-Religious".
Nevertheless, even the Soviets conceded that religious had served the causes of classes intent on progressive changes in the past (e.g. Christian opposition to Slavery) and efforts to use Christianity to express Socialism and Marxist beliefs can perform a progressive role in allowing the working class to articulate their views with their existing ideas. If you are a Christian and you are on the wrong side of capitalism, experiencing the injustices, exploitation and oppression, you are naturally going to draw on your existing ideas to understand those personal experiences. Within their ranks the Soviets had debates on
reforming the Russian Orthodox Church to be more Socialist and
whether Socialism entailed the establishment of new forms of religion, but they were controversial amongst Marxist-Leninists and generally rejected out of hand or tolerated based on an uneasy truce.
When you start to ask deeper philosophical questions, in practice trying to combine Marxism and Christianity, such as does god exist, was Jesus merely a mortal man or acting on behalf of a supernatural deity, will our prayers be answered, can you believe in evolution and the book of genesis, etc, you'll probably find that they don't work that well together and do conflict because they are based on very different understand on the nature of the world and what is possible to occur within it. But It is up to you to decide for yourself and to figure out what you really believe to be true.