Let me start with a short answer and explain it.
The short answer is this: time has no power over God, therefore time does not exist for God. If there is no such thing as time for God, then the concept of“beginning", which requires time, does not exist for God.
In Genesis 3, Moses has a conversation with God in the form of a burning bush. When Moses asked his name, God said,“I am who I am.” This, of course is not a name, but God revealing his nature. God us telling Moses that he is“esse ipsum subsistens" -- the sheer act of being itself. (fr. St. Thomas Aquinas) We understand this to mean God is an ETERNAL being.
See, God is a perfect being. This means nothing can be added or removed from him to make him more perfect. No such thing. Therefore we say God is unchangeable because there is no possible reason for him to change.Time is a measurement of change, so if time cannot measure God, we can say time has no power over God. This means God transcends time, that is, God is present in every moment of time at the same time. This is what we mean when we say he is eternal.
So time, as a concept, does not exist for God. Since“beginning" or“ending" is a particular moment in time, it cannot be applied to God. God is existence itself. He didn’t even create himself. He just always was, is, will be.
I know it can be difficult to totally comprehend but God is a mystery. This means we can know some things about God, but we cannot know everything about him.