It's obvious to any atheist that religious people, or at least followers of the Abrahamic religions in particular (Christianity, Judaism, Islam) are uncomfortable with them. One of the reasons, which I mentioned at the start of my chapter on "Morality for Atheists" in my book, is that religious people believe that atheists are dangerous because they have no moral code.
But there is, it seems, more to it, and this is an angle I had never thought of before: atheists frighten the religious because the presence of atheists, who have no belief in an afterlife, opens up the question, "What if they're right?" Theists don't believe we atheists are right, but they are unable to keep the seeds of doubt from germinating in their minds. And that makes them pretty scared of us, and makes them wish we would all go away. Or stop being atheists.
So says Tom Jacobs, in Pacific Standard, in an article titled "Why Atheists Terrify Believers". According to Jacobs, atheists "threaten the comforting narratives that give meaning to so many people's lives, and make the thought of death bearable."
I've given plenty of thought to the afterlife, and why so many people believe in it. The primary reason is that it is so difficult not to believe that our self-being will continue, in some other plane of existence, after we die. We can't imagine a universe in which we don't exist, because as soon as we try to visualize it, there we are in the middle of it, as the person doing the visualizing. (How do I get around this inability to imagine a world in which I don't exist? I don't bother trying.)
Yet every argument for the existence of an afterlife, in the usual sense of it, runs into inconvenient physical facts. I've got those facts summarized in my chapter titled "Some Questions". (It's the subject of question 4.) Essentially it's the observation that all parts of our selfhood -- our memories, our emotions, and even our sense of I-am-me -- are functions of the physical operation of the brain, each located in a specific part of the brain, and any one of those can be lost, permanently, when those parts of the brain are physically damaged. They are not independent of our physical existence. So if there is an independent "soul," it does not carry with it any of our memories, our emotions, or our sense of ourselves as individuals -- any of the things, that is, that we must, but are unable to, take with us into an afterlife following our deaths. Those are all things our brains provide for us, in a physical, electrical manner.
Interestingly, even a religion (Buddhism) without gods still assumes an afterlife, though it is rather a different perception of it. (I apologize to any Buddhists if I am misrepresenting their beliefs in any of what follows.) There are different branches of Buddhism, and the specific branch I am going to refer to here is Theravada Buddhism. Buddhism is a nontheistic religion (that is, not involving any gods, though it is unclear to me whether Buddhists think any gods exist). Buddhists, in common with several other Eastern religions, believe that we are all caught in a cycle of reincarnation -- that each of us, when we die, will be reborn into another (not necessarily human) life. Each of us, that is, is already in an "afterlife." (Exactly how the reincarnated me is "me" is going to need to be explained to me, since our next life comes without any memories of any previous ones.) In Theravada one strives to reach Nirvana, which is an internal state devoid of exactly those things that I said, above, that you can't take with you into an afterlife. In a state of Nirvana, you have no thoughts, no memories, no emotions, no sense of self -- or even a sense of time, so that the state, in a sense, lasts forever.
Nirvana is not an afterlife -- one achieves the state during one's lifetime. Obviously most people die without reaching Nirvana, and continue the reincarnation cycle. Nirvana is the hoped-for eventual escape from this cycle: in "Nirvana-in-this-lifetime," physical life continues, not that that matters to the living person in that timeless state. This is followed by "Nirvana-after-death," or "paranirvana," in which the body dies, disintegrates, and is not reborn. Paranirvana still isn't the afterlife Christians imagine. Buddhists, on reaching Nirvana and later dying, don't experience "life after death," since they do not exactly, in that state, "experience" anything at all. The self is gone. So when I say that Buddhists believe in an afterlife, I am referring only to reincarnation, not Nirvana.
Since Buddhism gets around so many of my objections to religion (belief in supernatural beings; belief in one's consciousness going on forever after death), I suppose I could almost become a Buddhist, except for my pesky need for evidence that a thing (such as Nirvana) can possibly exist before I'm willing to say it might. And there are also my objections to the cycle of rebirth, since, as I said, I am mystified by how the reborn me is "me" without any conscious sense of having previously existed. I am further mystified by the mechanism behind the rebirth cycle, in that no supreme being set it in motion or is in charge of it. Apparently it is simply the way things are -- with no beginning, going forever into the past, which is, today, at odds with all of the evidence science has collected that our planet, the life on it, and in fact the universe itself all had a beginning. But more importantly, there is zero physical evidence for the rebirth cycle. In regards to Nirvana, there is no evidence, because no one can describe it (since it is indescribable) and, obviously, no one has ever come back from it if they have attained it (and told the world, "Yeah, really, it was pretty cool"). So I'm going to put Buddhism in the same category as all other religions: "wish systems" based on unfounded beliefs.
My reason for mentioning Buddhism at all is to give an example of a belief system that is widely accepted into the classification of "religion" despite not having any gods, and to point out that it does, however, include a belief in an afterlife (though the goal is to escape it, not embrace it). So difficult is it for the human mind to avoid imagining an afterlife that even belief systems based in no way on gods still posit an afterlife. It seems that some sort of afterlife, rather than some sort of god, is the irreducible attribute of the human construct of religion.