First of all, this claim of atheists losing everything by going to hell forever is contingent on one interpretation of the Bible. There are a number of Christians out there who do not hold to the traditional view of eternal torment, but instead they ascribe to annihilationism. If such a doctrine happens to be the correct one over eternal torment then the end result will practically be the same: oblivion.
Second, it is undeniable that the end result of hell is by far infinitely worse than simple oblivion, however, that alone cannot be a basis for one to convert to faith, otherwise we would have to convert to every different faith which makes claims of hell fire.
Third, it is utterly false for the theist to claim that they have nothing to lose if they are wrong. Think of the time wasted praying everyday to a God that does not exist. A theist might respond and say that prayer, even if it is to a non-existent being, is still beneficial for one's mind. While this may be true, it disregards the fact that the accumulated prayer time of ones entire life could have been spent doing more productive activities, such as actually helping others. And in the process of helping others we are also helping ourselves because their joy and happiness also encourages and builds us up.
Then you have the time wasted going to church every sunday, or in some cases, several times a week depending on how dedicated a person is. This precious time wasted in a church building singing songs to a God that doesn't exist, reading and studying a fictitious book, could have been spent by planting trees, cleaning the environment, helping someone to have the best day ever. Try not to think of it in terms of a few hours every week wasted. It is easy to think, "People are only wasting a couple hours every sunday out of the entire week." because we can easily make light of that waste of time. But if we look at it from the perspective of the accumulation of the hours of a lifetime spent in church then we can begin to understand the full gravity of the tremendous waste which could have easily been prevented.
If theists knew that their religion was false, would they still continue to go to church and practice the religion? Or would they spend every single day doing what they love and getting as much out of life as possible? I suspect that most likely the latter will almost always be the case.
Then, we have the enormous sums of money that is donated to churches every week that go into the wallets of the church staff who are promoting these myths. Again, if we add the amount of money of a lifetime up that christians spend on their churches it probably would be enough money to feed entire nations.
Now, think of the lives that are unnecessarily lost for religion. People, in their zeal for religion, become missionaries and go to other countries and are put to death for their religion. So many lives are unnecessarily cut short because of false religion. If christians knew they were wrong, would they still continue to think that "we have nothing to lose" if they see missionaries in their 20's being martyred for a lie?
Theists try to justify the "we don't lose anything if we are wrong" idea on their current state of happiness, but there's a good chance that that happiness is contingent on the reality of their beliefs. Therefore, if they found out that they are wrong then they would consider their lives utterly wasted.
And finally, the amount of emotional and spiritual effort put forth for their religion is also wasted. Supposedly, Christians are in a war where, "their battle is not with flesh and blood but with the principalities, rulers, etc." They are in this spiritual battle against the devil who is constantly on the prowl like a roaring lion seeking whom he may devour. Christians spend a great deal of time battling these spiritual forces for absolutely nothing. As an alternative, which I am grateful to have discovered at the young age which I did; I live my life in peace and not constantly worrying about an angry wrathful God or by the darts of demons that are supposedly swarming around us.
Atheism teaches us to love and cherish life, not take it for granted since there is "something better in the next life". For me, there is no next life. We are here once, and most likely, never again.
