“The great argument for God was that there had to be a Creation, a beginning. … But my objection was simple. If God was the beginning who began God?” 
One of the most prominent assertion of metaphysics is that everything that begins to exist must have a cause of its existence. However, if something (or someone) never began to exist then a cause is not necessary.
God, according to Christian theism, is a timeless being beyond space, matter, energy, and time (transcendent), who is necessary in His existent, didn’t have a beginning and is therefore the first uncaused cause.
Or in the words of Oxford mathematician Professor John Lennox, “If you ask, ‘who created God?’ you’re assuming that God was created. Everybody knows that created gods are a delusion. We usually call them idols.”
Professor Lennox goes on to say, “It’s a question of is there an ultimate reality or do the questions go back forever. The fact is that on neither side do they go back forever. The Christian says the ultimate reality is God and by definition, you can’t get behind that. The atheist says that the ultimate reality is matter, energy, space, and time. So, the real question is not ‘is there an ultimate reality?’, the real question is ‘what ultimate reality is it? Of course I believe that the ultimate reality of God makes sense and the the ultimate reality of the universe doesn’t make sense.”
 Adams, P., The atheist delusion (an edited version of a speech Phillip Adams gave at the 2010 Global Atheist Convention); abc.net.au/news/stories/2010/03/19/2850137.htm.