Tillich, op. cit., pp. 20-21 -- "Philosophy necessarily asks the question of reality as a whole, the question of the structure of being. Theology necessarily asks the same question, for that which concerns us ultimatly must belong to reality as a whole; it must belong to being. Otherwise we could not encounter it, and it could not concern us. Of course, it cannot be one being among others; then it would not concern us infinitely. It must be the ground of our being, that which determines our being or not-being, the ultimate and unconditional power of being. But the power of being, its infinite ground or 'being itself,' expresses itself in and through the structure of being. Therefore, we can encounter it, be grasped by it, know it, and act toward it. Theology, when dealing with our ultimate concern, presupposes in every sentence the structure of being, its categories, laws, and concepts."Atheist philosophy asks nothing except how do I navigate living between birth and death and interact reasonably with my fellow inhabitants of this planet. Tillich does not get to redefine philosophy as the question of the structure of being, let alone the ultimate and unconditional power of being BS er God. It is pure and unadulterated theology with no more worth to an atheist than Yahweh, God, the Higgs Boson or the tooth fairy. None of which provide anything useful to my life except perhaps some lessons from myth. 'Being itself' doesn't even have any useful mythology or lessons. The entire lesson is have faith in being itself and feel good. It doesn't even get me from one minute to the next. I know how to feel good without bullshit.
Tillich's ultimate concern is indeed theology as it deals with that overriding meaning or "Ground of Being" in one's life. It has no more to do with an atheist's everyday living between birth and death than God, god, or any other ultimate concern. An atheist is aware of and compliant with the mores of herm chosen social support group, but there is no worship or ultimate concern about those mores simply the natural and genetic imperative of a social animal.
Much of this comes from reflections on an undergraduate religion seminar focused on Tillich conducted by a leading academic theologian Dr. Robert McAfee Brown. His Doctorate was in the philosophy of religion.