Almost all religions throughout history, no matter how old or new, claim to be either the first, the last, or both. This was as true of Zoroastrianism 2,000 years ago as it is of Judaism today. It's still as true of Christianity or Islam as it was of ancient Greek or Egyptian mythology. But isn't that always the line between a "religion" and a "mythology?" The simple passage of time since it was last practiced?
If we were to list every single religion that has ever died out through cultural shifts, genocide, factionalization, replacement, or absorption into other religions, you'd still be reading this by the time we got around to worshipping Borg space otters. Instead we're going to focus on a few "dead" religions that might not be quite as "dead" as you think. Because much like the Borg, new religions have a fascinating way of assimilating the old; absorbing and modifying concepts from long ago and far away, then forgetting where they originally came from.
But no belief system is the first, and no one alive today will likely survive to see the last. The things we believe today are but single links in the chain of human history. Some day, thousands of years from now, people will have trouble remembering when they started worshipping space otters. Otterism will feel like the first and last religion to them, too. But maybe, just maybe, someone will take a break from cracking clams on their tummy long enough to look back on Christianity, Islam, Judaism, Hinduism, Shintoism, and Buddhism. Maybe they'll see a bit of themselves in our "dead religions," just as you might see a bit of yourself in these.
Druidism Was Made Up by Nostalgic Brits in the 19th Century
You may know a few fans of paganism who would describe themselves as "Druids" today. But they aren't. First, because "Druid" wasn't a religion, it was a Celtic social class. A Druid was a member of the educated, professional class of Celtic peoples in France, Great Britain, and Ireland. They were teachers, philosophers, businessmen, and yes, priests. But they were priests of the same Celtic religions practiced by their contemporaries. And the simple fact is that nobody knows exactly what these people believed. Or if they even had a common belief system.
What we know of the religious practice today comes in part from the Celtic Revival of the 19th century, which saw Brits attempting to explore and return to their pagan roots. Some of the rest comes from the writing of Julius Ceasar and other Romans, who mythologized Druid priests in Gaul as sage, magical wise men, wandering the land with cloak and beard. Basically, Gandalf. But the best of our actual knowledge of Celtic or Druidic belief makes up only a small part of the modern Celtic belief structure. The vast majority of it was just kind of... made up, in the 19th Century.
Including that bit about Stonehenge, which we now know was built some 2,500 years before the Druids were even a thing. Of course, it still could have been aliens.
Japanese Emperor Worship Was Practiced Until 1946
It's not uncommon for ancient or medieval nations to position the monarch as some sort of divinity. Such was the case in Japan for most of the time when it had an emperor. This belief of Shintoism says that the emperor is an arahitogami, or human being who is also a god.
This belief only ended in 1946, at the end of the Second World War. After surrendering Japan to the United States, Emperor Hirohito signed the Humanity Declaration, stating that he had never been an arahitogami, and had simply inherited his title through family lineage. This was a critical step toward moving moving Japan out of its Imperial Age and into the modern age of democratic rule. By explicitly renouncing his divine status (and thus the divine status of all future emperors), Hirohito was no longer an imperial sovereign, but a constitutional monarch similar to the secular royalty of Great Britain or Canada.
Mithraism Was Christianity Before There Was Even Christ
This dead religion is incredibly interesting mostly because it isn't exactly "dead." It just became Christianity. Mithras was a very popular god worshipped by Romans during the 1st to 4th Century BC, during the initial formation of Christianity. Mithras was a demi-god who was born of a virgin on December 25th. He was a great teacher who traveled the land spreading wisdom, had 12 disciples, and was identified with both the lion and the lamb. Romans called him "Good Shepherd," "Redeemer," "Savior," and "the Way, the Truth and the Light." All of which you may recognize from the New Testament - most of which was written well after worship of Mithras had already begun.
Mithras was even buried in a tomb, and resurrected after three days. The Romans celebrated his resurrection on the feast day of the female goddess of fertility. Depending on the specific region, that would be Aphrodite, Ishtar, or Astarte, from which we get the word "Easter." Sounds like a pretty solid case that Jesus was really Mithras, right?
Maybe - or maybe not. Because Mithras himself was based entirely on the Egyptian demi-god Horus. Every single thing said here applies as much to Horus as Mithras and Jesus. In addition, Horus was born under an Eastern star, was attended by three wise men, walked on water, healed the sick, was baptized at 20 years old by "Anup the Baptizer," and was later represented by the ichthys fish symbol adopted by Christians.
Care to guess Horus's birthday? Here's a hint: You celebrate it every year... on Christmas.
Canaanite Polytheism Consolidated Its Many Gods to Become Judaism
Funny - don't see many Canaanites around these days. Whatever happened to those guys, anyway? Maybe the Amalakites know. In any case, Caananite polytheism (or Baal worship) was once a pretty big deal in the Middle East, and it lives on at least to some extent in the religion that ultimately replaced it.
Fans of the Old Testament know all about Baal and the Canaanites, but what fewer people know is that "Baal" wasn't a single god. It was just a title, meaning "Lord" or "God." There were many Baals worshipped in this area, the highest of whom was called El. As in "El Elohim," "Yahweh," or "the exact same god of the Old Testament."
The Old Testament also mentions other gods. Example: "Thou shalt have no other gods before me." See, originally, the Hebrew god El was a Canaanite Baal - the father of all other gods, like Asherah (also mentioned in the Old Testament), Astarte, Dagon, and many others. Think of El as equivalent to Zeus, Apollo, or Odin; not a singular god, but an all-powerful father to the others.
When the ancient Hebrews came through, they were flying the banner of the "god of all gods." Effectively, the Hebrews just cut out the middlemen and decided to worship the Baal El directly. Likely under the assumption that whatever Dad says goes among his kids, too.
This means two things. First: Baal worship is still alive and well in every church, synagogue, and mosque on Earth. Second: Judaism was not purely monotheistic. It was simply a version of pantheist Baal worship that places emphasis directly on the most powerful god. If that's your measure of "monotheism," then any temple devoted to Zeus, Apollo, or Odin could say the same.