Religions that never were - but might be
Hymn to Mithras, sung by the XXX Legion stationed at the Wall (Hadrian's) north of Eboracum (modern York).
Mithras, God of the Morning, our trumpets waken the wall!
Rome is above the Nations, but Thou art over all,
Now as the names are answered, and the guard is marched away,
Mithras, also a soldier, give us strength for the day!
Mithras, God of the Noontide, the heather swims in the heat,
Our helmets scorch our foreheads; our sandals burn our feet,
Now in the ungirt hour; now lest we blink and drowse,
Mithras, also a soldier, keep us true to our vows!
Mithras, God of the Sunset, low on the Western main,
Thou descending immortal, immortal to rise again!
Now as the watch is ended, now as the wine is drawn
Mithras also a soldier, keep us pure till the dawn!
Mithras, God of the Midnight, here where the Great Bull dies,
Look on thy children in darkness. Oh take our sacrifice!
Many roads Thou has fashioned: all of them lead to the Light,
Mithras, also a soldier, teach us to die aright!
I don't know about you, but that hymn to an extinct religion always sends chills down my spine. The worship of the solar deity Mithras, the "soldiers' god", was once the most serious rival to Christianity. The Christians ultimately co-opted several features from it, such as December 25 as the birth date of the Savior (originally the winter solstice before calendar reforms altered the relationship with the seasons) and Sunday as the Sabbath, rather than the original Jewish Sabbath of sundown Friday to sundown Saturday.
Mithraism lost out to Christianity, probably for a couple of reasons. One was that the Mithraic mysteries were reserved for men. This resulted in Roman households where the women were Christians and the men Mithraists. When the primary caregivers are of one faith, you can pretty much guess which is going to win out in the long run.
Another was that the Mithraism insisted on a high moral standard for candidates for admission - a Christian bishop once bitterly remarked that, "The Devil shames us with the quality of his adherents." The Christians would take you as you were and work on upgrading your morals. Nonetheless there is something very compelling in that vision of Roman legionaries singing to their god, asking for strength to fulfill their duty of guarding the civilized lands against the northern savages (i.e. my ancestors) and a poignancy that comes from the knowledge that eventually their strength failed and they were overwhelmed.
Now here's the rub, very little is known about Mithraism, that hymn was written by Rudyard Kipling as part of his 'History of England' series.
Mankind invents new religions, and variations on old ones all the time, and existing religions have schisms like cats have kittens. We are a religious animal, and there's no escaping that. Religion is at least as old as mankind and I don't see humans becoming indifferent to religion in the foreseeable future.
But what is it going to look like? That I wouldn't take any bets on.
As Kipling invented a hymn for a religion in the past, many science fiction writers have invented religions for the future. One, L. Ron Hubbard actually got serious about it and founded Scientology. Other SciFi writers have done far better in my opinion, but didn't go so far as to take their creations seriously enough to proselytize for them.
Robert Silverberg took the idea of a "religion of science" and in my humble opinion, did a more appealing job of it in his novel 'To Open the Sky'. He postulated a religion which worshipped the mysteries of "the quantum, and the holy angstrom" in the Litany of the Wavelengths and sought immortality through scientific research rather than life-after-death.
Poul Anderson created at least two religions. In novels such as 'The People of the Wind' he created a race of intelligent birds, the Ythrians (as if humans had descended from hawks rather than primates). The deity of their New Religion was called God the Hunter.
So what kind of religion would a race of flying hunters create? Their god is a hunter - and we, all living beings, are his prey. We exist to give honor to god. God loves us, the way a hunter would love the prey in his sights. Our obligation is to fight as hard as we can to live as long as we can, so that god has honor from us.
Sound chilling? Yet Anderson wrote a very moving eulogy for this religion, "High you flew on many winds, until at last God stooped on you in your pride. Long you fought Him and well, and from you He has honor. Go now. Be wind, be ash, be water. Be always remembered."
In the same future history series he invented, or adapted, the religion he called Cosmenosism (See: The Day of Their Return). Some variation of this actually seems to be emerging among people who can't buy into faith-based supernaturalism, but still feel the religious impulse strongly.
The premise here goes something like, rationalist attempts at a definition of God often look a lot like a self-aware universe. So without supernaturalism, how does a universe become self-aware? By evolving life and intelligence. Matter organizing itself until one day a living being looks around as says, "I exist!" Intelligent beings further evolve, naturally and by developing their science and technology until they are so powerful and wise that they are pretty much indistinguishable from what we'd call gods.
This is cool, because it gives the atheists a way to have God too. Many variations are possible. Have other races made the journey to transcendence before us? That is, is God waiting for us to join him, and maybe lending a helping hand? Or do we become God far in the future, but are able to reach back in time to help ourselves up? Are we in fact going to become immortal?
Scientist Frank Tipler posits a future where our supercomputers will give us immortality by recreating in emulations, not only all human being who ever lived, but all human being who ever could have lived. Others speculate that if the universe is an expanding and contracting one, at the point where it starts to contract, all information will become available to us, including the information that went into making each and every one of us.
It's interesting to note that something like a variation of Cosmenosism is the core theology of Mormonism. Other variations look something like the Hindu belief in cycles of creation.
What might be considered another variation is the crypto-Buddhist philosophy of Viriditas in Kim Stanley Robinson's Mars trilogy: Red Mars, Green Mars and Blue Mars. The core principle here is that life and living worlds are so rare and precious, that our duty as sentient beings is to terraform and bring life to as much of the universe as we can, throughout our future existence as a race.
But what if it's all a sham? What if belief is something we invent to hide from ourselves the fact that the universe is indifferent to us and someday we're all going to die and be - nothing. George R. R. Martin invented the Liars, in his short story 'The Way of Cross and Dragon'.
In this story, an Inquisitor for a far-future Roman Catholic church charged with the duty of fighting heresy, meets a heretic who tells him, "I'm a Liar." "I know you're a liar" he replies. "No, you don't understand, I'm a Liar."
The heretic tells him that he is a member of an underground sect called the Liars. They believe that there is no God and no afterlife, but that the vast majority of humans can't live with that knowledge - so throughout the ages they invent comforting religions, creating mythologies tailored to the specific cultural needs of each time and place. The Inquisitor vanquishes the new heresy but is left with nagging doubts about his own faith. In the end he requests to be relieved of his duties because he has lost his faith. His superior coldly informs him that faith is not necessary for him to fulfill his duty...
John Maddox Roberts also saw the Roman Catholic church continuing into the far future, in his delightful novel 'Cestus Dei', which is Latin for "The brass knuckles of God". (The Cestus was a kind of boxing handwrap, often with shot or spikes attached, used by a class of gladiators called pugilists or cestiarii.)
Cestus Dei is an order of Jesuit martial artists. At one point a potential convert tells a member of the order that understanding is easier for him because he grew up in the Faith. "But I didn't" the Jesuit tells him. He informs the young man that he grew up on a planet settled by Hindus, and was a worshipper of the goddess Kali, of a sect that strangled men as an act of religious devotion - Thugee! (The cult of the stranglers in India, origin of the English word 'thug'.)
He tells the young man that he found the faith when he saw a Christian missionary on the street of his city and, thinking that the killing of an infidel would be pleasing to his goddess, followed him with his silk rumal (scarf) with the intention of strangling him. "I woke up in the hospital a week later. As my bones healed, the Jesuit visited me every day and explained to me the truth of the Faith."
It's a hoot!
Verily, many and marvelous are the ways of God and Man.