Yeah I get what you mean its pretty easy to mistake as im pretty sure marx was an atheist who just sympathized with religious people but honestly if I were you id just ignore idiots like this who blame anything they dont like as being the fault of the devil.
Marxism is atheist because Marx hated God and Jesus Christ; Probably because Jesus said this, “A new commandment I give to you, that you love one another; as I have loved you, that you also love one another."