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.
Catharism Was Brutally Eradicated by the Catholic Church
Those who know the history of the Catholic Church are already well familiar with the Cathars. Specifically, because they don't exist anymore.
Catharism was a gnostic religion. The Cathars believed there were two gods: the evil God of Creation spoken of in the Old Testament, and the good God of the New Testament. Essentially, the Cathars believed that the Yahweh of the Old Testament was really Satan, who had created every single physical thing in the world, including people. They believed that humans were essentially genderless, angelic spirits trapped within a body made by the Devil, and that only knowledge of the truth ("gnosis") could relieve the angelic spirit of its constant reincarnation into evil flesh.
By 1209, the Catholic Church had mercilessly wiped this entire group of heretics off the face of the Earth. Only the Black Death killed more Europeans than the Catholics during this genocidal purge. Today, we remember the Cathars on one specific date, the day on which the last Cathar fortress fell. The Church burned over 200 Cathars alive at the foot of that castle. Cathars called this day "The Massacre of Montsegur" - today we simply know it as "Friday the 13th."
Zoroastrianism Invented the Ideas of Heaven, Hell, and Monotheism
This fascinating Babylonian religion may not be well known today, but it's probably one of the most important religions of all time. Especially for modern Jewish and Christian culture. In fact, the more you learn about this supposedly dead religion, the more you may come to recognize some of your own beliefs within it.
Appearing approximately 2,000 years before Jesus, Zoroastrianism far predates the earliest extra-biblical mention of Judaism. In Zoroastrianism, we find history's first verifiable mention of a supreme being of wisdom, love, and enlightenment. His name was "Ahura Mazda." This makes Zorastrianism effectively the first monotheistic religion. In it, we find history's first mention of a "hell" where evil people suffer for their bad deeds, and a "heaven" created for good people.
It's worth noting that these concepts, so commonly believed by Christians, Jews, and Muslims, are actually nowhere to be found in any of their respective religious texts. You'll find nothing in the Quran or Bible that says people go to heaven just for being generally good, or hell just for being generally evil. This convention held by so many Abrahamic believers is strictly Zoroastrian, handed down orally by reformed Jews after the Babylonian Captivity. Zoroastrianism is a fascinating case of oral tradition surviving in parallel (or even direct conflict) with written tradition.