When you say, “I believe in God,” you mean that you know with absolute 100% certainty that God exists. However,
Job doesn’t use the word “believe” in that way. Even if he says, “I believe that I exist,” he only means that he is about 99.9999999999% certain that he exists, but he is always open to that 0.0000000001% chance that he is wrong. He understands that, outside of abstract fields like mathematics and logic, he can never know anything with absolute 100% certainty.
I have a friend who once said, “Jonathan is going to win this game. 100 percent!” Then, Jonathan did not win that game. It seemed to me that my friend’s worldview had been completely shattered. Something that he knew with absolute 100% certainty was false. I couldn’t see how he could trust himself at all ever again. He would have to go insane.