Most atheists respond to the Christian statement that God created the universe with a question of ‘Well then, Who created God?’. This is really a school boy argument that has been around since ancient history.
But the issue here is that atheists are thinking of a created God and not about a God that has always been present. The Christian worldview is that God was always there. Look at the first few verses of the gospel of John:
1 In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was with God in the beginning. 3 Through him all things were made; without him nothing was made that has been made. 4 In him was life,and that life was the light of all mankind. 5 The light shines in the darkness, and the darkness has not overcome it.
There is nothing logically wrong with proposing that God was always there – in fact, non-theistic woldviews have largely proposed that the universe was always there until proof in the 20th century came out overwhelmingly that the universe was created.
Dr. Lennox puts this much better in his sermon on The Testimony of Science: http://www.rzim.org/let-my-people-think-broadcasts/the-testimony-of-science-part-1-of-2/