You can't. I wouldn't even try. I personally believe in God on the basis of my subjective experience.
Which doesn't make it invalid; it just makes it non-transferable. If you look around you can find lots of empirical evidence, but none of it... not even all of it in sum... adds up to the theoretical proof you're looking for.
One of the more compelling subjective evidences you have to consider is your sense that there is objective Reason. I share that sense.
Regards!