To use a feeble analogy, it is rather like asserting that it is inadequate to say that light is the cause of illumination because one is then obliged to say what it is that illuminates the light, and so on ad infinitum. 

The most venerable metaphysical claims about God do not simply shift priority from one kind of thing (say, a teacup or the universe) to another thing that just happens to be much bigger and come much earlier (some discrete, very large gentleman who preexists teacups and universes alike). These claims start, rather, from the fairly elementary observation that nothing contingent, composite, finite, temporal, complex, and mutable can account for its own existence, and that even an infinite series of such things can never be the source or ground of its own being, but must depend on some source of actuality beyond itself. Thus, abstracting from the universal conditions of contingency, one very well may (and perhaps must) conclude that all things are sustained in being by an absolute plenitude of actuality, whose very essence is being as such: not a “supreme being,” not another thing within or alongside the universe, but the infinite act of being itself, the one eternal and transcendent source of all existence and knowledge, in which all finite being participates.