What was God, before being God? Was he the pleroma? 

No, not the pleroma [a term from Gnosticism], if that is what you are thinking. By definition, God is uncaused. He never came into being. Only things which have come into being (like the universe, as Big Bang cosmology has demonstrated) require causes. So asking about something that existed before (outside of) time is a meaningless question, since there is no "before" before there is time! 

For more on this, take a peek at my first (2007) debate with Scientific American's Michael Shermer on DVD. Or, if you don't mind missing the 45-minute Q&A, you can watch the whole thing at YouTube. My starting point is the Big Bang and its space-time implications.