If God is real, why doesn't He just do something blatant to reveal Himself, like speak from the Heavens or appear to everyone on Earth?
Much of the evidence we demand is related to who we think God is and what His purposes are. If God's purpose was for everyone to simply acknowledge His existence, then He likely would supply more evidence, but God wants much more than that. If God were to prove Himself, an atheist would become a theist, but not all people would become Christians. Knowing God exists does not translate to loving Him and trusting Him. Proof isn't all we need - proof is external, but we need something internal.
If all our demands for proof were met, we would only trust God to the extent that He proved Himself. We would be relying on external evidence and our judgment, not trusting God Himself. All other relationships are founded upon trust - the bank loaning money is a risk to them and requires them to trust you, allowing a friend to borrow and return a personal item requires risk and trust, and even trusting your spouse to remain faithful and keep their vows is a relationship founded upon trust. God requires us to trust Him because trust builds a deeper relationship with the one trusted.
If God were to reveal himself to creatures with freedom of thought, anything He could do to show His existence could be refuted. We would rationalize that we were hallucinating or dreaming, that it was an illusion, that it wasn't really God, or that it was someone masquerading as God. Since many of these objections can't be completely disproven, God's existence can't be absolutely proven. Even if God did appear to every man, not all would believe in Him. God did reveal Himself to mankind, speaking like "no man ever spoke" (John 1:1,14; 7:46), performing miracles, healing the blind, reattaching severed body parts, and raising the dead - yet, many still did not believe in Him (John 11:45-53; 12:9-11). Though He appeared to them and performed miracles before them, people rejected Him. Even if some were to acknowledge His existence, many would deny His authority over them, just as Judas did. Even before God revealed Himself to man, there were a number of angelic beings who knew God yet rejected His will for them (2 Peter 2:4; Jude 6). Many of us would choose sin over the Savior even if He appeared to us.
Though God hasn't given us proof, He has given us everything we need to believe (Romans 1:18-20). Since the creation of the world, God's invisible qualities have been clearly seen and understood from what has been made. Throughout history, our ancestors have asked questions like "How did the stars that guide us get in the sky?" How does the rain produce for us a harvest? Is there someone who created and controls these things? If we are not afraid to ask, and if we are humble enough to seek Him, He will give further evidence that we may need in order to believe (Matt. 7:7-8).
So why doesn't God prove He exists? In short, because He chooses not to. Though we don't know all of His reasons, that does not mean he doesn't exist or that He is unfair. I think Philip Yancey's quote describes God's character regarding this subject well - Yancey says, "More amazing is His refusal to perform and to overwhelm. God's terrible insistence on human freedom is so absolute that He granted us the power to live as though He did not exist, to spit in His face, to crucify Him. I believe God insists on such restraint because no pyrotechnic displays of omnipotence will achieve the response He desires. Although power can force obedience, only love can summon a response of love, which is the one thing God wants from us and the reason he created us."
God loves you and longs to be in a relationship with you, but He will not force Himself upon you. If the cross where He was crucified and the tomb that was left empty aren't enough evidence for you, ask Him to reveal Himself to you. Try opening His word with an unbiased mind, willing to let Him speak to you through it. If you are unwilling to look for evidence for His existence, then ask yourself, do you want Him to exist? Are you biased?
