How do we rationally prove the existence of God? With dominoes, of course!
Someday, when I figure out how to make gifs, I will supply a gif here, but for now, please use your imagination.
I lay a small mat flat in front of you.
I place four dominoes on the mat, standing up ready to fall.
The mat and the four dominoes are their own little system; their own little universe if you will. Watch this system/universe awhile and you will see nothing happens. Nothing happens because within the little system/universe, nothing causes anything to happen.
Then reach out and topple the first domino, causing them all to fall over.
The fourth domino fell down. Why? Because the third domino caused it to fall.
The third domino fell down. Why? Because the second domino caused it to fall.
The second domino fell down. Why? Because the first domino caused it to fall.
The first domino fell down. Why? Because and not until you caused it to fall.
You were an uncaused cause from outside the little system, the little universe of the mat and the dominoes.
Nothing happened in the little system until an uncaused cause from outside the system got things started. In the same way, nothing happened in our big universe until an uncaused cause from outside the universe got things started.
Think about one chain of causality in our universe.
The sky is blue during the day. WHY?
Because light comes from the sun and gets scattered by the atmosphere. WHY?
Because the sun produces light from constant nuclear explosions. WHY?
Because hydrogen was floating in space and the sun gathered so much hydrogen that the hydrogen explodes constantly. WHY?
Hydrogen was floating in space because the Big Bang put it there. WHY?
The Big Bang put hydrogen in space because an uncaused cause caused the Big Bang.
There MUST be an uncaused cause of the universe.
We call the uncaused cause of the universe God.
Speaking rationaly, because the universe exists, God MUST exist.