Most Ancient religions don’t have a founder.This includes not only Hinduism (which you’re referring to as “Sanatana Dharma”) but also the Ancient religions of the Mesopotamians, Egyptians, Chinese, Persians, Greeks, inhabitants of Asia Minor (now known as Turkey), Norse, Celts, Shinto, Canaanites and Arabs.
The tribal religions of Africans, Native Americans and Oceanians don’t have founders either.
Most such religions are characterized by multiple gods and goddesses (polytheism). But Judaism (yes – it doesn’t have a founder), is the only clear-cut exception of being a monotheist religion since its inception in Israel (which is a newer nation compared to the afore-mentioned ancient civilizations).
According to history, all these religions emerged when humans began to worship the forces of nature as personified deities. Hence, they don’t have founders.
On the other hand, religions which branched out from other religions have founders.
This includes Buddhism, Jainism and Sikhism which have branched out from Hinduism.
Buddhism has only a single historical Buddha, i.e., Gautama Buddha. But it’s also mentioned in the Pali Canon that prior to Gautama Buddha, there were at least three previous Buddhas in the present Kalpa.
Jainism has two historical Tirthankaras viz., Parshvanatha and Mahavira. But the Jain tradition holds it that the religion originated with Rishabhanatha, a non-historical Hindu king who was said to have set up the foundational principles for the practice of Jainism (hence, he’s regarded as the first Tirthankara of Jainism). These principles became a religion apart when Rishabhanatha’s son, Bahubali decided to relinquish his kingdom for the sake of his brother, Bharata, and thus became the first Jain monk.
Rishabhanatha and Bharata were mentioned as Vaishnava Hindu kings in Hindu scriptures such as the Bhagavad Purana. But no mention has been made to Bharata’s brother, Bahubali, in Hindu scriptures.
Now, unlike Buddhism and Jainism which are alleged to have existed even before the existence of the historical founders, but are still traced to a non-historical founder, the creation of Sikhism lies entirely under the purview of history. Guru Nanak, the founder of Sikhism, is a confirmed historical figure and so are the nine other Sikh Gurus who succeeded him.
Even though Ancient Persians originally revered numerous deities and supernatural beings known as Ahuras, Yazatas and Daevas, it was Zoroastrianism which eventually emerged as the dominant faith in Persia over the years. Zarathustra, the founder of Zoroastrianism, believed that Ahura Mazda was the only God in the cosmos and that He was immanent within everything in life as the principle of Goodness within every soul. Zarathustra was opposed to polytheism and so were his followers.
Zoroastrianism became more popular when Persians conquered Assyria, liberated the Jews from Assyrian rule and allowed them to return to Israel. The Jewish influence strengthened the cause for the wider adoption of the monotheistic Zoroastrianism. According to the Bible, Xerxes, the Persian Emperor at that time, was a Zoroastrian and he married Esther, a Jewish woman. This marriage depicts how Jews and Zoroastrians found similarities with each other’s culture.
It has to be noted that whenever a new religion is founded from the original polytheist ones, it’s either a non-theist religion like Buddhism or Jainism or a monotheist one like Sikhism and Zoroastrianism. This implies that new religions are usually founded in a spiritual resistance against the existing polytheistic establishments.
Likewise, Taoism and Confucianism in China, both of which are non-theist, have branched out from the polytheistic culture of the Chinese religion which revered numerous divine beings known as Shen, all of which operated the forces of nature (Tian) under the Mandate of Heaven (Tian Ming). However, very less information is available on how the Chinese religion has evolved due to the lack of transparency in the details of the Chinese history as well as the hostility against organized religion that characterizes the present-day Communist rule in China.
Taoism is traced to Lao Tzu and Confucianism is traced to Kong Fu Tzu (Confucius).
In Japan, Shintoism, which is the ancient polytheistic religion of Japan, does not have a founder. But Tenrikyo, which believes in the immanence of a single deity, is remarkably founded by Nakayama Miki – a woman – while on the other hand, most of the world’s religions with founders were founded by men. It’s also interesting to note that the single deity in Tenrikyo is also female. Goddess.
While most religions without founders are typically polytheist in nature, Judaism, the first Abrahamic religion and the mother of all Abrahamic religions, is said to not have a founder.
In fact, the Bible claims that there has only been One God all the time and that other deities and religions were eventually invented by tribes who became rebellious against the One God over unfulfilled desires that they started dreaming that there were other deities who were capable of fulfilling these desires. Hence, these people were doomed to an eternal sentence in Hell because of that God’s insecurity over the humans’ act of worshipping stones that He supposedly becomes jealous of these stones.
While Jews made no attempts to spread their religion out of Israel, yet habitually considered other civilizations inferior to them, Christians and Muslims who adopted these same beliefs from Judaism, went on a campaign of cultural erasure wherever they went out of the dogmas they adopted due to the lack of scientific and rational knowledge.
In reality, Judaism is actually a new religion compared to the Ancient religions of Egyptians, Greeks and Mesopotamians around them. There’s no historical proof for Adam, Noah, Abraham, Lot, Jacob, Joseph or even Moses. That’s why it’s said they’re mythological figures, and not historical ones.
There’s only historical proof that Jews had been slaves for some ancient nation, most probably Egypt, before they somehow freed themselves wholesale and built their civilization in the land of Israel. However, Israel was already occupied by an idolatrous tribe known as Canaanites. Hence, the Jews fought a war against them and destroyed them. They even destroyed their places of worship including temples and idols before occupying their region.
According to the Bible, these Jews were led by Moses and he was commanded by God to destroy these temples of Canaanites and their idols. Then God told Moses that He was the only God in the cosmos and that no other god should be worshipped other than Him. He also told Moses that idols should not be made or worshipped.
However, according to history, the Jews chose to consciously worship only one God to avoid sectarian divides that were the hallmark of the polytheistic civilizations of the nations around them. Besides this, the Jews’ monotheism is a rebellion against the established polytheistic norm of all ancient religions and was hence a learned trait, and not an authentic one.
Even the Jews’ avoidance of idol-worship was about centering their worship towards a single location – towards the Ark of the Covenant which is a wooden box containing the original stone tablets of the Ten Commandments received by Moses from God. This wooden box was eventually later consecrated in the Temple of Jerusalem by King Solomon. Ever since then, Jews faced the direction of the Temple of Jerusalem during worship.
While polytheists had multiple holy places due to their beliefs that each consecrated idol in each particular location was holy, Jews had only one central holy place. This mechanism was also meant to unite Jews and did not pertain to any mystic Truth. Rather, since Jews were a new nation, they had to adopt dogmas against polytheism and idol-worship in order to be safe from the Ancient imperialist aggressors of Egypt, Mesopotamia and Greece.
Sadly, these dogmas were taken as historical truth by Jews in the Ancient times. Hence, when Christians and Muslims adopted the same dogmas, they themselves became aggressors and went on a campaign to erase all cultures except their own.
The only serious attempt to correct the historical mistakes of these Christians and Muslims is the Baha’i Faith, which, like Hinduism, taught Unity of Religion and even made attempts to rationalize the bigoted notions of Eternal Hell held by Christians and Muslims.
While Judaism did not have a historical founder, but evolved as a monotheistic religion due to the historical context in which the freed Jewish slaves occupied Israel after fighting a war against Canaanites, the foundation of Christianity is traced to the teachings of Jesus Christ (who did not have any intention to create any new religion) while the foundation of Islam is traced to the conscious efforts of Muhammad to create a new religion based on whatever he learnt from Halima, his wet-nurse, who was a monotheist as an adherent of the hanif sect of the Pagan Arab religion.
Needless to say, Baha’i was founded by Baha Ullah who had wider knowledge of the World’s religions than Moses, Jesus, Muhammad or any other Abrahamic prophet.
Now, just because a religion does not have a founder, it does not mean that it’s good or original. Nor does a religion with a founder have to be bad or artificial.
Whether your religion is good or bad depends on what its God teaches you to do.
If your God teaches you to practice the Dharmic principles of Truth, Love, Mercy, Forgiveness and Peace, then that God is a good God and you’re in a good religion.
But if your God teaches you to do the exact opposite of these Dharmic principles, your God is evil. So, you’ll have to abandon this religion for some other religion.
However, if you want to stay in the same religion, you have to find a better teacher or a better source of knowledge from your religion through which you can learn the Dharmic principles of Truth, Love, Mercy, Forgiveness and Peace. By doing this, you can reform your God (or rather, your idea of God).