Why vaishnavism,shaivism and shaiktism have their own head god?
Originally this started back from vedic period diffrent -diffrent hymns were created in the glory of its god. In rigveda there was vishnu sukta for lord vishnu In same rigveda there was devi sukta in praise of devi. Rudra suktam also was in rigveda. But the satarudriya of yajurveda is also a great hymn to god rudra.
But vedas also assested that these all gods are one and of no image.
This started and lord narayan in taitrayi aranyaka of yajurveda was considred as purush of vedas.
And in same taitrayi aranyaka place lord rudra was also attested to be the purush of vedas and all that exists.
And we can say that apart from drivercity the vedas and taittrayi aranyaka both have given equality of lord narayan and lord rudra. Same as vedas given equality of all gods.
Please read the article for further more information:—
RUDRA AND VISHNU's EQUALITY
PROVED IN VEDAS
Sameas in post-vedic texts like ramayan and mahabharat the diffrenciation was at average level although some time they show both the gods as diffrent peesnality but then showed their equality.
Here's the proof:—
These two, viz., Brahma and Rudra, are the foremost of all the deities, having sprung respectively from the Propitiousness and the Wrath (of Aniruddha). Acting according to Aniruddha's direction, these two deities create and destroy. Although capable of granting boons unto all creatures, they are, however, in the matter of the concerns to which they attend (viz., Creation and Destruction), . instruments in the hands of Aniruddha.
— SHANTI PARVA,SECTION CCCXLII
Here is another proof:-
Thou art he that hadst created from thy right side the Grandsire Brahma, the Creator of all things. Thou art he that hadst created from thy left side Vishnu for protecting the Creation. Thou art that puissant Lord who didst create Rudra when the end of the Yuga came and when the Creation was once more to be dissolved. That Rudra, who sprang from thee destroyed the Creation with all its mobile and immobile beings, assuming the form of Kala of great energy, of the cloud Samvartaka (charged with water which myriads of oceans are not capacious enough to bear), and of the all consuming fire. Verily, when the period comes for the dissolution of the universe, that Rudra stands, ready to swallow up the universe. Thou art that Mahadeva, who is the original Creator of the universe with all its mobile and immobile entities.
— The Mahabharata, Book 13: Anusasana Parva: Anusasanika Parva: Section XIV
Harivamsa parva says about this :-
rudrasya paramo viShNurviShNoshcha paramaH shivaH |eka eva dvidhA bhUto loke charati nityashaH ||2-125-41
The supreme lord of rudra (shiva) is viShNu and the supreme lord of viShNu is shiva (rudra). The same lord is moving in the world always in two forms.
But by the starting of puranic age gods were started to be mapped as supreme on their purans or the other god as second best. although oldest purans like vayu purana and matsya puran does not do so. As vayu puran was present at time of mahabharat.
The Vayu Purana is mentioned in chapter 3.191 of the Mahabharata, and section 1.7 of the Harivamsa
And both vayu purana and matsya puran gives creation story as
Bhraman divided himself into bhramha,vishnu and shiva.
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