Written 17 October 2011; minor revisions 16 September 2018
Here's how I think science will eventually put religion out of business. Soon medical science is going to be able to offer serious life extension, not pie-in-the-sky soul survival or re-incarnation, but real life extension with possible rejuvenation. When science can offer and DELIVER what religion can only promise, religion is done.
1) Religion is in the transcendence business. The type of transcendence offered depends on the particular religion. The highly sophisticated form of Christianity expounded by Thomas Aquinas offers the visio beata, the Beatific Vision. In the BV -- you will forgive the abbreviation -- the soul does not lose its identity. It maintains its identity, though in a transformed mode, while participating in the divine life. Hinduism and Buddhism offer even more rarefied forms of transcendence in which the individual self is either absorbed into the eternal Atman/Brahman, thereby losing its individual identity, or extinguished altogether by entry into Nirvana.
And there are cruder forms of transcendence, in popular forms of Christianity, in Islam, and in other faiths, in which the individual continues to exist after death but with little or no transformation to enjoy delights that are commensurable with the ones enjoyed here below. The crudest form, no doubt, is the popular Islamic notion of paradise as an endless sporting with 72 black-eyed virgins. So on the one end of the spectrum: transcendence as something difficult to distinguish from utter extinction; on the other end, immortality mit Haut und Haar (to borrow a delightful phrase from Schopenhauer), "with skin and hair" in a realm of sensuous delights but without the usual negatives such as heart burn and erectile dysfunction.
I think we can safely say that a religion that offers no form of transcendence, whether Here or Hereafter, is no religion at all. Religion, then, is in the business of offering transcendence.
2) I agree with my correspondent that if science can provide what religion promises, then science will put religion out of business. But as my crude little sketch above shows, different religions promise different things. Now the crudest form of transcendence is physical immortality, immortality "with skin and hair" and with no moral or spiritual transformation. Is it reasonable to hope that future science will give rise to a technology that will make us, or some of us, physically immortal? I don't think so. That would violate the Second Law of Thermodynamics according to which the entropy of an irreversible process in an isolated system increases leading in the case of the universe (which is both isolated and irreversible) to the heat death of the universe and the end of all life. Granted, that is way off in the future. But that is irrelevant if the claim is that physical immortality is possible by purely physical means. And if that is not the claim, then the use of the phrase 'physical immortality' is out of place. In a serious discussion like this word games are strictly verboten.
3) Physical immortality is nomologically impossible, impossible given the laws of nature. Of course, a certain amount of life extension has been achieved and it is reasonable to expect that more will be achieved. So suppose the average life expectancy of people like us gets cranked up to 130 years. To underscore the obvious, to live to 130 is not to live forever. Suppose you have made it to 130 and are now on your death bed. If you have any spiritual depth at all, your lament is likely to be similar to that of Jacob's: "The length of my pilgrimage has been one hundred and thirty years; short and wretched has been my life, nor does it compare with the years my fathers lived during their pilgrimage." (Genesis 47:9)
The important point here is that once a period of time is over, it makes no difference how long it has lasted. It is over and done with and accessible only in the flickering and dim light of intermittent and fallible memory. The past 'telescopes' and 'scrunches up,' the years melt into one another; the past cannot be relived. What was distinctly lived is now all a blur. And now death looms before you. What does it matter that you lived 130 or 260 years? You are going to die all the same, and be forgotten, and all your works with you. After a while it will be as if you never existed.
The problem is not that our lives are short; the problem is that we are in time at all. No matter how long a life extends it is still a life in time, a life in which the past is no longer, the future not yet, and the present a passing away. This problem, the problem of the transitoriness of life, cannot be solved by life extension even if, per impossibile, physical immortality were possible. This problem of the transitoriness and vanity of life is one that religion addresses. It is one that science cannot address, let alone solve.
So my first conclusion is this. Even if we take religion in its crudest form, as promising physical immortality, "with skin and hair," science cannot put such a crude religion out of business. For, first of all, physical immortality is physically impossible, and second, mere life extension, even unto the age of a Methuselah, does not solve the problem of the transitoriness of life.
4) But I have just begun to scratch the surface of the absurdities of transhumanism. No higher religion is about providing natural goodies by supernatural means, goodies that cannot be had by natural means. Talk of pie-in-the-sky is but a cartoonish misrepresentation by those materialists who can only think in material terms and only believe in what they can hold in their hands. A religion such as Christianity promises a way out of the unsatisfactory predicament we find ourselves in in this life.
What makes our situation unsatisfactory is not merely our physical and mental weakness and the shortness of our lives. It is primarily our moral defects that make our lives in this world miserable. We lie and slander, steal and cheat, rape and murder. We are ungrateful for what we have and filled with inordinate desire for what we don't have and wouldn't satisfy us even if we had it. We are avaricious, gluttonous, proud, boastful and self-deceived. It is not just that our wills are weak; our wills are perverse. It is not just that are hearts are cold; our hearts are foul. You say none of this applies to you? Very well, you will end up the victim of those to whom these predicates do apply. And then your misery will be, not the misery of the evil-doer, but the misery of the victim and the slave. You may find yourself forlorn and forsaken in a concentration camp. Suffering you can bear, but not meaningless suffering, not injustice and absurdity.
Whether or not the higher religions can deliver what they promise, what they promise first and foremost is deliverance from ignorance and delusion, salvation from meaninglessness and moral evil. So my correspondent couldn't be more wrong. No physical technology can do what religion tries to do. Suppose a technology is developed that actually reverses the processes of aging and keeps us all alive indefinitely. This is pure fantasy, of course, given the manifold contingencies of the world (nuclear and biological warfare, terrorism, natural disasters, etc.); but just suppose. Our spiritual and moral predicament would remain as deeply fouled-up as it has always been and religion would remain in business.
5) If, like my correspondent, you accept naturalism and scientism, then you ought to face what you take to be reality, namely, that we are all just clever animals slated to perish utterly in a few years, and not seek transcendence where it cannot be found. Accept no substitutes! Transhumanism is an ersatz religion that cannot deliver.
It could be like this. All religions are false; none can deliver what they promise. Naturalism is true: reality is exhausted by the space-time system and its contents. You are not unreasonable if you believe this. But I say you are unreasonable if you think that technologies derived from the sciences of nature can deliver what religions have promised.
As long as there are human beings there will be religion. The only way I can imagine religion withering away is if humanity allows itself to be gradually replaced by soulless robots. But in that case it will not be that the promises of religion are fulfilled by science; it would be that no one would be around having religious needs.