Eternal Universe
Atheists argue that God is not required because an Eternal Multi-verse (many Universes) could be a possible alternative. So just like God is eternal, they argue that the Mutliverse could also be forever without a beginning.
The Philosophical Reply:
Imagine there are an Infinite amount of people Ahead of You in a line. Will you ever get your turn to reach the front?
Similarly it does not make sense for the Multi-verse to cause this Universe to have a beginning. Since –according to that argument – this Universe would never have a chance to have a Beginning* (as there would be infinite amount of Universes before this one could come into existence.)
*According to the Big Bang, this Universe is around 13.5billion years old.
God does not need a Beginning;
This is why when atheists ask believers who created God – it does not make sense. We believe God is not created by a series of infinitely regressing created gods, otherwise this God would have infinite amount of ‘gods’ before He could ‘come into existence’.
So instead we affirm that God is One, the Originator, without being created, outside the boundaries of our space and time.