A common atheistic response to the question "what if you're wrong" is to simply flip it. There are more religions out there than just Christianity, after all. As it turns out, I have actually thought about it. Obviously, not with regard to every religion, but a lot of them. With Evolution, the what if I'm wrong is simply it doesn't matter. My beliefs in this life do not affect anything, either in my life, or after my death, so if I'm wrong about Evolution, so what?
Not so with Islam. What if I'm wrong about that? In this scenario, my answer is that, if I was allowed to maintain my current mindset, I would ask Allah to bring to my memory every Qur'an verse, every Haddith tradition, and every Bible verse I'd ever read. Then, I would proceed to explain the Islamic dilemma to him.
The Qur'an and the Haddith actually have a lot of positive things to say about Christianity. They say that the Torah was given to the Jews, that the Gospel was given to Jesus, and that neither group of people have any ground to stand upon unless they judge by these books. It even asks why the Jews came to Muhammad for judgement when they had the Torah. And yet, obviously, Islam is a very different religion to that described in the Bible. Therefore, Islam cannot be true, because it is contradictory, and so if Allah is the real god, it would be 100% his own fault I was a Christian in life. Thus, my bid for Heaven (or at least for avoiding Hell) with Allah would simply be "I did what I thought would please you". If he still condemned me for that, what right or ability would I have to resist?
But obviously, simply because of Islam's contradictory nature, I have a pretty good idea as to its falsehood, and if a god's religion is false, it's not likely to be the real God.
What atheist can do this kind of thing with the Bible? The God of the Bible is a very just God. We even see that on Judgement Day, it will be possible to talk to Him. Matthew 7:22 says "Many will say to me (Jesus) on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?" (NIV, brackets added). So when Jesus judges you on judgement day, it seems you can talk to Him and try to justify yourself, even if you are one of those to whom He says "I never knew you, depart from me evil doer". So, if you're an atheist, and you're wrong, Christianity is true, 1. Jesus will judge you, and 2. you have a good chance of being able to plead your own case, much like in my hypothetical Allah scenario, I would try to plead my own.
So, return to the original question. Atheists, what if you're wrong? Jesus isn't an obscure god, so sorry Richard Dawkins, the "Great Juju at the bottom of the sea" argument doesn't work very well. Jesus is right there in front of your face, so you can't plead ignorance. You have no excuse for not thinking about Him, especially if you're then going to have the nerve to blaspheme Him on a regular basis.
Jesus also isn't a contradictory god, like Allah. There is no "Christian dilemma". You could say "well there's the problem of evil", but that so-called problem has been addressed from 360 different angles throughout history. The answer is so simple, and so easily accessible, that even if you didn't have the equivalent of all the world's libraries at the tip of your fingers, even your local church probably has at least one semi-competent apologist in every pew.
Perhaps you would rely on the "I'm a good person" defence. But tell me, are you? Perhaps you think you are. But read God's laws. How many have you broken? How many have you justified breaking? Only one good man has ever walked the earth. The rest of us fall short of God's Holiness, and so He must punish us, even for the sins we consider "small".
But maybe you'll challenge God's Holiness. Maybe you're like Matt Dillahunty, "if I go to Hell, I'll go knowing I was morally superior to God". Ok, really? What makes you morally superior to God? "Well I don't like His morals". Well He doesn't like yours either. Given that He has the supreme of every attribute, including omniscience (i.e. He has all the facts) I'd say He wins. Especially since, you know, He owns the universe He created. You literally have no right to tell Him He's wrong, because 1. He isn't, 2. He knows it, and 3. it's entirely His call.
So let's hearken back to my Allah strategy for a moment. I'd ask Allah to bring to my memory everything I knew about Islam and Christianity, and from there, he'd see not only that I was a sincere Christian, but also why. But could you do this with God? There are some people in this world who genuinely could plead some degree of ignorance. This, the Scripture tells us, will reduce their punishment. But if you're here, you're almost certainly not one of them. You know things. You probably have some knowledge of the Gospel. You almost certainly have a high degree of knowledge about the evidence for Christianity. If God reminds you of all relevant knowledge you have, will He find that you were a sincere atheist, and why? Or would He find a lot of things you may well have been deceived by, but willingly so? Judging by the quality, or lack thereof, of just about every atheistic source on the planet, no atheist who has ever lived will be able to utter a word in their defence.
Of course, morally, that would always be the case. Every single one of us is a sinner. There is no difference between a Christian and an atheist in that regard. Thus, there is only one way to survive judgement day if Christianity is true. The only good answer to "what if you're wrong" is you are wrong. Stop it. Look at the evidence you have been graciously afforded by the God who owes you nothing, but bought you everything. Repent. Because there is nothing worse you could possibly be wrong about.