I dunno about Catholicism but I've toyed with the idea of doing entryism at small churches. They're pre-existing institutions with a low barrier to entry and also one of the last vestiges of community in a lot of the US. It likely wouldn't be useful to engage with franchised ones, but local ones might work. Think Damnation.

Do you think anti-religion is fundamental to Marxism

I don't think Marxism is a religion, it's an analytic method. Thus it can be adapted in all sorts of ways, including ways that accommodate cultural beliefs. Whether that's useful or not is debatable and dependent on the situation.

Whether Lenin believes so is highly relevant to his situation but requires actual analysis to cross-apply to another. Otherwise it's just book worship.

But personally I'm an atheist.