In the yogic tradition, Shiva is not worshipped 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 himself into the Adi Guru.
The first Guru was born and the Adiyogi started teaching the Saptarishis, his first seven disciples. Guru Purnima marks one of the greatest moments in the life of humanity. This is a day that is reminiscent of Adiyogi opening up and offering the possibility that a human being can evolve beyond all limitations set by his physical nature, if willing to strive.
This predates all religion. Before people devised divisive ways of fracturing humanity to a point where it seems almost impossible to fix, the most powerful tools necessary to raise human consciousness were realised and propagated. Every possible way in which you could approach and transform the human mechanism into an ultimate possibility was explored thousands years ago.
The sophistication of it is unbelievable. The question of whether people were so sophisticated at that time is irrelevant because this did not come from a certain civilisation or thought process. This came from an inner realisation.
This had nothing to do with what was happening around him. It was just an outpouring of himself. In great detail, he gave a meaning and a possibility of what you could do with every point in the human mechanism.
You cannot change a single thing even today because he said ev.