I've heard this sentiment quite a number of times over the years (and several times in the past week). Sometimes it is the claim that Christians don't really believe they are going to heaven, sometimes it is that they don't believe in the afterlife at all, and sometimes it is that they don't believe in anything about God they claim to. Sure, they have convinced themselves at some surface level, they believe it enough to put on a good show, but deep down, they really know there is no God, no heaven, nothing supernatural going on. These kind of statements have always bothered me.
The arguments usually examine the way the religious handle certain situations and point out that if they "really believe" they would act differently. Often it is when discussing death. They will claim Christians should not fear death, because it just means they are going to heaven. They should not be sad when their friends and family die, because they should know they are now in heaven and much better off. The argument goes that since Christians are typically very sad in such situations, they must not really believe that their friends are going to heaven. Since they are afraid of death they must not really believe in heaven (or at least that they are going there). I hate it whenever I hear this kind of argumentation.
Life is complicated, human nature is complicated, and emotions are complicated. It is possible to be simultaneously looking forward to something new while clinging to something old. Anyone who has ever gotten a better job or has moved to a new exciting place knows this. The unknown is big and scary, even exciting unknowns. You are comfortable where you are, your friends are there, who knows what it will be like when you get there? You are optimistic but still part of you doesn't want to go. If your friend is moving to a new job you are happy for them but still sad they are leaving. Why would heaven be any different? Granted, moving across the country to a new job is much different than going to a perfect existence in heaven, but I imagine the emotions could be analogous. If your best friend is going to heaven, you can be happy for them but also sad that you will never see them again. To jump to the conclusion that they are sad because they don't believe in heaven is asinine to me.
But that is just one possible explanation why the behavior of the Christian in this discussion is understandable, I am sure there are others. What really bothers me about seeing this argument from atheists is they are projecting their worldview onto others. They look at the way others act and project their worldview onto them to explain the action. They fail to really try to understand how that action could be explained with an alternate world view. It doesn't make sense to them that you could believe in heaven and be scared of death so they proclaim that the person doesn't believe in heaven.
This is the same thing that Christians do to atheists when they insist that we are simply rebelling against God. "Why do you hate God?" or "Why are you angry at God?" These are questions I have heard from Christians and they bother me as well. They believe in God and can't understand that I don't believe in God so they assume that I do. This is just Atheists doing the same thing in the opposite direction. I don't like it either way, we should try to understand each other rather than put words in each other's mouths.