Question - what existed before God?
Short answer
There has never been a "before" God. God is eternal (Deut. 33.27) and the one who simply "is" (Exo. 3:14) . By definition that means that God doesn't have a beginning nor an end.
Long answer
This answers runs the risk of turning into a reflection on whether or not God exists. The long answer includes a discussion on the proof of God's existence called "The contingency proof". It shows that God must exist because we exist and that God therefore has to be eternal, which means without beginning or end.
1. Everything that has a beginning must a a reason, or cause, for coming into existence. Nothing can casue itself to come into existence. The result can't be greater than the cause.
2. The universe had (by all accounts) a beginning.
3. That means that there must be a cause behind and beyond the universe's existence.
4. This cause must "have" existence in itself/himself/herself and the ability to pass it on to something else, namely the universe.
5. This means that something/someone must exist outside the universe and that this something/Someone must not be a created thing. Something or someone that/isn't created must be eternal. Therefore, since the universe exist God must exist and since God is therefore outside of time and space He must be eternal.
And then Jesus...
He is the image of the invisible God, the firstborn of all creation. For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him. And he is before all things, and in him all things hold together.
(Col 1:15-17)