Without God in your life, you have nothing to look forward to. Atheism leads to hopelessness and despair.
For many people, their religion teaches that their god gives them purpose in life and offers them something to look forward to after they die something that is important because, so often, these religions teach adherents that their actual lives right now aren't of much value. Therefore, anyone who doesn't follow this religion and who doesn't believe in their god must not have purpose in life and must not have anything to look forward to. All of this is wrong, not to mention arrogant.
The belief may follow logically from the premises which believers use, but there is a problem: the people who are reaching these conclusions are relying upon assumptions about and stereotypes of atheists, not any interactions with real atheists. This myth is often repeated by someone who "sees what they want to believe." You don't find any real interest in learning about atheists and agnostics because they assume they already understand all about atheists and agnostics not through personal research, but because of their own religious assumptions.
As a consequence, even when real information about real atheists is encountered, it often isn't believed or accepted at face value. Prejudices and assumptions are given a preferential position over and above disconfirming facts and evidence. I receive quite a lot of email along these lines: someone feels sorry for me, assumes that I dont have anything to live for, insists on letting me know that Ill be prayed for, please accept Jesus into your heart, etc., etc., etc. None of it is very original, but all of it can be quite arrogant.
To start with, it is arrogant as well as mistaken to assume that, without a belief in their god, then I must have nothing to hope for. Granted, I cant hope for their particular idea about an afterlife, but that hardly warrants the conclusion that I have nothing to hope for or to look forward to in the future.
What do I have to look forward to? Life an enjoyable life doing the things I love and being with the people I love. Why do I live? Because of the people I love and the things I love basically, because I enjoy life. Does it matter that, eventually, I am going to die and the life I enjoy will end? I admit that that will be unfortunate, but it doesn't mean that doing what I enjoy now is therefore worthless. After all, every individual action I am doing will end every good meal end, every trip to an amusement park ends, every good book ends.
The same is true for religious theists and Christians, but I don't see many of them eschewing experiences that must necessarily end on the basis of the belief that non-eternal experiences aren't worth having. Since these religious theists don't actually behave as though they believe this principle is true, it's difficult to accept that really believe this myth or the argument that atheism should be rejected because it leads to hopelessness.
Does the fact that every moment ends and cannot be recaptured mean that, therefore, it was worthless and meaningless to experience and enjoy that moment? Not at all. There is absolutely no reason to think that the temporary nature of any particular moment or experience forces us to de-value that moment or experience and, therefore, to also de-value all of life.
Although it's rare to find any theists eschewing non-eternal experiences, it isn't unusual to find them devaluing life and our experiences in life as compared to the "eternal" existence they anticipate having after their physical deaths. Because this life is temporary and the assumed afterlife is eternal, only the afterlife is truly valued while this life is de-valued. At most, this life is given value as preparation for reaching the preferred afterlife. It is not, however, valued in and of itself. This life is not valued for what it is and for the experiences it offers.
Because of this, the questions these theists ask atheists should really be turned around: if this life with all of its temporary experiences is of so little importance while the afterlife with all of its eternal-ness is the real prize, why do they bother to go on living this life? Why would they want to be here eating, drinking, reading, and sleeping when all of that must necessarily end when they die? It makes much more sense to hasten their journey to their real goal: the eternal afterlife, where everything has value because it is eternal rather than temporary.
It is interesting to note that this myth about atheists contradicts another popular myth, namely that atheists are only interested in sex and other forms of physical, material pleasure. On the one hand the absence of god and religion supposedly causes atheists to have no reason to care about living; on the other hand, the absence of god and religion also supposedly causes atheists to spend too much time enjoying themselves with worldly pleasures. Which is it?
I have no need for an afterlife, a heaven, a god, or any supernatural beliefs in order to appreciate living and take joy out of my life. For many religious theists, and for Christians in particular, it is unacceptable to think that life on earth is all that we have. To me, that sounds like someone who doesnt take much joy out of their life. A person who truly enjoys and appreciates their life will take pleasure in it and enjoy it regardless of whether any sort of afterlife exists. They might believe in an afterlife and even in some sort of wonderful heaven, but they wont depend upon the existence of such a heaven in order for their lives to have meaning or purpose. The disagreement here is thus not between theists who believe in an afterlife and atheists who do not, but rather between theists who are unable to find value in this life and everyone else atheists as well as theists who can.