The proof is in what God chooses to reveal. Indeed God has revealed himself to mankind,
For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: (Romans 1:20 KJV)
but mankind refuses to look at the evidence.
Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. (Romans 1:21 KJV)
What evidence are you referring to?
The man who wrote these words claims to have been revealed to the Reality of the Eternal God. That's all the evidence he would need.
I had the same type of revelation, though not near as elaborate as Paul's "road to Damascus" account.
But I wouldn't have just taken his word for it if I hadn't seen it myself.