Atheists don’t need to have a single philosophy because atheism is not religion, they don’t have a common book to refer to and have come mostly to their conclusion by thought rather than teaching.
Moved to Religion. Perhaps the title could be re-stated to help in answering the question?
Does every Christian have their own philosophy? If so, why is that a problem? What makes you think Christianity has to be or is supposed to be a closed system? What is a Christian anyway? Last time we discussed that here, there was no consensus about what is meant by the term. Everyone who calls herself a Christian does not agree about the definition of God either.