I would like to suggest that our attitude to death might be the final give-away. I don't just mean death in general, but specifically our own mortality.
Of course there is a broad spectrum of hopes and beliefs, and in a short blog post I can only speak in very general terms. I think is possible to narrow down our attitudes to four main points of view and I'll try to place these on Richard Dawkins' spectrum of beliefs. (From page 50 of The God Delusion; 1 is a strong theist, and 7 is a strong atheist.)
1/ You know that there is a heaven and you are confident that you will get there because even if you are not perfect, you are repentant and you know in your heart that this is true.
3/ You don't think about it often, but when you do you are pretty sure that there is an afterlife and you fear that you might go to hell and suffer eternal torment. You don't fear this quite enough to change all your life decisions, but it does nag at you. But still . . . there is always time to mend your ways and to repent later. (Or is there?)
5/ You don't think about it often, but when you do, you still have a lingering sense that you have a spirit or soul, and you would like it to live on after your mortal life is over. You want this in some unspecific spiritual sense, since you don't really believe in the gods that others worship. However, you are still horrified at the concept of your own mortality.
7/ You know that there is no 'hope' of an afterlife and find this viewpoint to be quite soothing - even therapeutic. After the highs and lows of a life lived well (or badly) you will neither be subjected to the continuous joy of paradise (which must get boring eventually) nor to the eternal torments of hell. After death there will be perfect peace and your entire being will cease to be. And you really don't mind! You think "when I'm gone, don't grieve for me, but look after the people who loved me".
Now I know that the words are slightly skewed towards what I might call 'Christian atheism'. By that I mean the kind of atheist who has rejected the idea of the Christian God and ignores even the existence of the other options. However, I think it might work for Muslims if the words were changed a little, and for other religions to varying degrees. In the case of Buddhists this scale tests something other than atheism, since they are all atheists, but I think a Buddhist apostate could aspire to reach 7, with practice.
Religions clearly foster the human desire for an afterlife. It is one thing that almost all of them have in common. Indeed the afterlife might be the carrier of the virus of religion.
If I score 7, does that mean that I'm not enjoying my life enough? What do you think?