As I explained before is the concept of trinity not confined to the christian religion. This could have been a surprise for some readers. So I give an overview here.

The clearest concept of trinity is found in Hinduism, where it is called trimūrti (त्रिमूर्ति), which means “three forms” in Sanskrit and means concretely the three forms of brahman (ब्रह्मन्), the universal, immanent and transcendent, eternal and unchangeable divine background of everything. These three forms are the gods Brahmā (ब्रह्मा) or creator, Viṣṇu (विष्णु) or maintainer, and Śiva (शिव) or the destroyer and re-creator.

The three highest gods of the ancient Greek and Romans were Zeus/ Jupiter, Hades/ Pluto, and Poseidon/ Neptune. The three are brothers. They have three sisters, so they are altogether six siblings. Female deities do quite often appear in threefold form. This is due to the simple biological cause that women have an only limited and rather short phase of frugality in their life. So many goddesses appear in groups of three, only sometimes sisters, where one represents the frugal phase, one the time before and one the time after the frugal phase. This is why the three goddesses of fate, Klotho, Lachesis, and Atropos are female. This could well be the origin of the idea of trinity. The ancient Greek had also three goddesses, who are the Moon. The young and hence virgin Artemis, the frugal Selene, and the old and hence no longer frugal Hekate, who became later the goddess of witchcraft while Artemis became goddess of the hunt.

But back to the three brothers. If you have watched the Percy Jackson movies then you saw how Hades was shown with kind of huge fork with only two tines. A symbol of his power. You can say that it is a two-headed scepter. Poseidon/ Neptune is famous for his trident, also a symbol of his power, so a three-headed scepter. Zeus/ Jupiter has such a symbol of his power too and it is his flash of lightning, at which you can look as a one-headed scepter. Besides the three gods being brothers, these scepters, which show their rank and power, additionally show that these three gods belong together. There scepters were not only in ancient Rome and Greece the symbol of divine power. Śiva (शिव) is also well-known holding his trident. There are Hindus, who pray only to Śiva (शिव), and are called Shivaites. These use three horizontal stripes to show their religion. Viṣṇu (विष्णु) has similar followers, who are called Vishnaites and use two vertical stripes to show their religion. Shivaites and Vishnaites are the two groups, which often clash violently at Hindu festivals. Brahmā (ब्रह्मा) seems not to have such followers nowadays. The important point here is that Śiva (शिव) is represented by a trident like Poseidon/ Neptune or three stripes. So Viṣṇu (विष्णु), who is represented with two stripes, is similar in rank to Hades/ Pluto, while the rank of Brahmā (ब्रह्मा) is then of course left for Zeus/ Jupiter. The three gods of the ancient Greek and Romans do thus resemble the trimūrti (त्रिमूर्ति) in their symbolism. Hence it isn’t a reach to say that Zeus-Hades-Poseidon as well as Jupiter-Pluto-Neptune were at least at some point in history representing the holy trinity.

Ancient Roman authors, when they wrote about other cultures, often explained religious believes in the way: they call this god by that name. For example: “They call Apollo by the name Balder.” or “Their main god is Hercules, who they call Thor.” They didn’t completely get it because then the Romans wouldn’t have equaled the god of thunder with the half-god Hercules. They important point here is that the religions were so similar that it was possible to give explanations this way. The Germanic religion did of course know the holy trinity. Originally it was represented by Wodan-Wili-We, one god known with different aspects under three names. But the concept deteriorated. The gods were seen as separate, Wili and We disappeared as names, while other names appeared. It became a kind of challenge in poetry to describe the same gods with different names. The only aspect of the pantheon, which wasn’t changed, was the idea that there should be a trinity ruling over the pantheon. Thus Odin, Thor, and Tyr formed the final trinity before this religion vanished. The closely neighboured Celts called their trinity by the names Teutates, Belenus, and Esus. In Sumeria the gods Enlil, Ea, and Anu formed the holy trinity.

Sometimes ancient religions had more than one name for one god and sometimes was a position in the trinity exchanged with another god. This could depend on the geographical region or the period of history. Most such changes can be retraced in ancient Egypt. There the pantheon was generally divided into groups of three. But presiding over all such groups was the trinity of Osiris, his wife Isis, and their son Horus. Osiris had at that point already made a colorful career. Before was his wife Nephthys and their son Anubis, but Anubis was also listed with a different father or a different mother. Before that was Osiris’ wife Hathor and their son again Horus. Osiris was god of the underworld, but only at the end of his career, before he was a Sun god and before this a frugality god. The pair of Hathor and Osiris were frugality gods. Frugality was brought to ancient Egyptians by the floods of the river Nile and Osiris was at the beginning of his career a river god. Back then he was shown in the three colors blue, black, and green. The three-colored Osiris was the beginning of the Egyptian idea of trinity. Considering the long history of ancient Egypt, its power and influence, many other cultures will probably have taken their concepts of trinity from ancient Egypt. Like the Babylonian trinity of Baal, Ishtar, and Tammuz. This could like the trinity of Osiris-Isis-Horus have influenced the christian idea of trinity if they aren’t even the origin of the concept of trinity of the christian doctrine.

Even in religions, which aren’t teaching the existence of any god, there is still a tendency to arrange their saints into groups of three. On the altar of a Buddhist temple do you usually find a big statue, accompanied by two smaller statues, placed left and right of the big statue. The same do you find on altars of Confucian temples.

The only religions in the world, which explicitly deny the existence of a trinity are Islam and modern Judaism. Although God is called by different names in the Bible, like YHWH and Elohim. What else could these names be meant for than representing different aspects of the divine? Thorough readers even realize that the use of different names of God in the Old Testament is dependent on God’s actions. Mystics of all three Abrahamic religions are sure that there are seventy-two names of God, a quite large number, which can be split into 3²x2³, so performing some numerological operations with the number three. Although Islam and Judaism deny, as the only religions on Earth, the holy trinity, they are nevertheless prepared for developing some acceptance for it.