Divine Hiddenness

[John 6IX Breezy]

“Just as God's purpose does not allow man to be a physical prisoner, neither does it permit him to exist in an intellectual prison. How would man behave if God were to constantly reveal Himself? Would he really be free? If man were constantly made aware that he was standing in the King's presence, could he go against His will? If God's existence were constantly apparent, this awareness would make man a prisoner.“

As I understand it, belief in God's existence has never been the most important variable:

"You believe that there is one God. You do well. Even the demons believe—and tremble!" (James 2:19, NKJV).

And in one of the parables the analogy was given of a dead man asking to be sent back from the grave so that people might see him and repent. The response was this:

"He said to him, ‘If they do not hear Moses and the Prophets, neither will they be convinced if someone should rise from the dead" (Like 16:31, ESV).

Obviously this is New Testament and your friend is Jewish, but it echoes his point in a sightly different way. It argues that nothing of value changes by believing He exists. Almost every story in the Bible, from Adam and Eve onwards, involves people who saw or experienced God in some way, and nevertheless betrayed Him.

So this adds to your friends argument, which I think is good as well.