"You say your God can do anything. Have Him say hi". These words were displayed over an image of an empty chair in a meme I once saw an atheist post. This is a very common argument. God, they claim, does not exist, not because of any evidence they have found, but because of this one specific piece of direct evidence that they haven't.
While this argument may distress inexperienced Christians, particularly if they are young, there are a number of ways to respond to it, starting with the fact that Christians can't "have God" do anything. Not only does God have free will, He also has the absolute authority to use it in any way He sees fit. Thus, while it is quite within His power to show up directly, He is entirely capable of withdrawing from us and making us rely on the rest of the evidence instead.
This, by the way, includes times when He did show up. See, while atheists, in their arrogance, demand God show up again, they fail to recognise that there have been times all throughout history when He has. What's particularly ironic is that the more obvious He makes Himself, the more people beg Him to go away again. "And they said unto Moses, Speak thou with us, and we will hear: but let not God speak with us, lest we die." (Exodus 20:19). And off God went, and suddenly, they apostatised again. Funny that, isn't it? It is just the nature of man.
When God is near, we fear Him. When He is absent, we act as if He's absent. The sad thing is, many atheists even think if God did show up, they would speak to Him as if He was a man. They certainly won't, of course. Every atheist, when they finally meet God, will be in awe and dread of Him, finally knowing who they have sinned against.
But of course, for now, He doesn't often appear to atheists. They must instead focus on other evidence. And that is completely ok. It's logical to believe in people before you meet them. You've never met my brother, but you probably wouldn't bat an eye when I tell you I have one. It's not 100% necessary that I have a brother. There are many people in this world with no siblings. Yet, it's believable that I have one, and my testimony that I do is sufficient. In the same way, it is perfectly acceptable to accept the testimony found in Scripture. Even ignoring the element of divine inspiration, Scripture is a composition of numerous eyewitness accounts. Moses was there when God lead the Israelites through the desert as a pillar of smoke by day and fire by night. Paul was there when Jesus appeared to Him and converted Him. It's logical to accept their testimonies.
While it is not necessary that I have a sibling, it is necessary that I have parents. I am not eternal, I was not created from the dust of the Earth, I didn't just form from a magical explosion. If you were to say something as stupid as "let me meet your mother, then I will believe she exists", I will be sending the police to your house, because you're clearly using some illegal drugs. In the same way, the universe is not eternal, nor is it the result of some magical explosion. It has to have come from somewhere. And that somewhere must have a lot of attributes which, conveniently, God has.
In the end, the fact that an atheist cannot bait God into personally showing up on command is a sad excuse for atheism. The evidence we do have is overwhelming. Romans 1 tells us that unbelievers are "without excuse", because creation itself makes Him known to us. You can't have creation without a Creator, and we are a very special creation living inside another particularly intricate creation. And whether we like it or not, every day, we rely on our Creator to keep it that way.
But there will come a time when God will show Himself to every eye, and every knee will bow to Him. Atheists should not be in a hurry to speed up that day, because if He comes to them while they remain in their rebellion, He will repay their treason. But as scripture says, "...be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day. The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance. But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up." (2 Peter 3:8-10).
God owes no man salvation, but He is both able and willing to give it. Every atheist is on the path to Hell, but as long as they remain on the Earth, they have the right to step right off it. Repent, confess Jesus as Lord, and believe He rose from the dead, and you will be saved.