Proving that something exists is simple. Just show where it is.
Proving that something does not exist is not as simple as it might seem. First, you investigate all you know. After you have explored all you know, and all that anyone else knows, and found nothing, you have only one more thing to do. Investigate the unknown.
Wait a minute. How do you investigate the unknown? The answer is, you can’t. As much as you might expand your knowledge, even to the ends of every detail in the universe, you cannot escape the idea that there might be something more beyond that.
Some atheists or agnostics will use the following demonstration: “God, if you exist, strike me down, now!” When God does not oblige by striking them down, they rest on the proof that God does not exist.
Suppose I make an analogy: I tell you all about my friend Bob, and what a great guy he is. Since you never have met Bob, and he sounds a little eccentric to you, maybe you do not choose to believe I have a friend named Bob. Maybe you don’t think I deserve such a friend. Or maybe you think it’s out of character for me to have any friends! So you decide to put an end to my fakery by calling out to the sky, “Bob, if you exist, come punch me in the nose!” Since Bob does not show up, you confront me with the fact that there is no Bob, and ask me not to speak of him again.
What did you do wrong? You didn’t use the proper mode of communication to reach Bob. Had you asked me before your “test,” I could have told you that Bob is visiting his aunt, and gave me her number: 555-6789! You could have contacted Bob, and believed in him.
It is similar in the case of asking God to strike you down. God is willing to prove his existence to you in many ways, but asking him to upset the natural order of the world he set up is not in His usual repertoire. The way Saul was converted is a notable exception. But most of us receive some small reassurance or answer to prayer to help us on the road to faith. That’s what you should ask God for. A small sign that would seem insignificant to others, but which would be proof to you, because it came very soon after you asked for it.
Throughout the gospels, Jesus makes it clear that God fulfills our requests based on faith. Asking vainly to be struck down by God, expecting nothing to happen, is rewarded according to its measure of faith.