[ Adiyogi ]
In the yogic tradition, Shiva is not worshiped as a God. He is the Adiyogi, the first Yogi, and Adi Guru, the First Guru from whom the yogic sciences originated.
The first full moon of Dakshinayana is Guru Purnima, when Adiyogi began the transmission of these sciences to the Saptarishis, his first seven disciples.
This predates all religion. Before people devised divisive ways of fracturing humanity, the most powerful tools necessary to raise human consciousness were realized and propagated. Their sophistication is unbelievable. The question of whether people were so sophisticated at that time is irrelevant because this did not come from a certain civilization or thought process. This came from an inner realization. It was just an outpouring of Adiyogi himself. You cannot change a single thing even today because he said everything that could be said in such beautiful and intelligent ways. You can only spend your lifetime trying to decipher it.
▵ 10 Forms of Shiva Explained (Isha)