You could make a case that the universe is eternal – in which case God may not be necessary. However, your premise would fly in the face of scientific evidence showing the universe to be expanding. So, presuming the universe had a beginning, something or someone without a beginning (i.e. eternal) had to have caused it.
You could also argue that something caused ‘the cause’ of the universe. But then the question becomes, ‘what caused that cause?’. Eventually, there has to be a first cause that has no beginning and is therefore eternal.
We are defining that first cause as God. Furthermore, we believe the necessary characteristics of that cause are consistent with those of a
God.