I have never met a happy atheist. I believe in separation of church and state, but I think we have gone so far over in the other direction of separating church and state.
If you become great, then you can become happy. If you're happy first, it's much more difficult to be great.
The assertion of failure coming from such persons does not mean that Mr. Mill failed to promote the practical success of those objects the advocacy of which forms the chief feature of his political writings.