Most atheists insist that if God would just appear to them they would believe. If He could just stand before them they could finally put to rest all of their doubts and truly bury the question "Does God exist." It's very easy to empathize with this. Wouldn't it be much easier if God would just appear before unbelievers and convince them once and for all that He was real? Wouldn't that make trying to convince them easier? At first this may seem like a good possibility but then the reality sets in as you begin to ponder back through Scripture. You could go back to Moses when he received the big ten from God Himself and no sooner was he gone, conversing with God with a multitude of witnesses below, were those witnesses constructing a giant golden calf to worship. There they stood with some kind of vision of the Lord, although veiled to some degree, and didn't believe and arrogantly making their preferred idol. Skip forward to the parable of the rich man and Lazarus in Luke. The rich man, in torment, asks if Lazarus, who was carried away in death by angels, could come to him and dip his finger in water to cool the flame. When this isn't a possibility, the rich man pleads for Lazarus to go to his five brothers and warn them to not go to this place of torment. But he is told they have heard Moses and the prophets so they should know already. But the rich man insists that if they see a man raised from the dead come and tell them, they will surely be convinced. However, the parable states that if they aren't convinced by the words of Scripture, not even a man raised from the dead and speaking directly to them could do it. How could this be? I would think that would be the most convincing evidence possible! But another remembrance comes to mind - the very people that walked with the physical Jesus could not even recognize His resurrected body!
I think the human nature of self-righteousness rears it's ugly head here. We actually think enough of ourselves that by seeing Him we would believe. If you can't recognize Him in Scripture, you would look right past Him even if He stood directly in front of you. Scripture shows this time and time again. If you can't look at the wonders of His creation in your children or in the beauty of nature that surrounds you, His physical appearance will do nothing to you.
This offers no comfort to the atheist because they MUST see to believe. But you'll never see Christ if you first don't see your need for Him. You need no proof if you are good enough as you are. Christ doesn't seek the good, He seeks the sinner. You'll never see Him if you are good. I have all the proof I require for the existence of God - me and my clothes of unrighteousness, the flesh.