Markandeya is an ancient sage in the Shaiva tradition and the primary teacher in the Markandeya Purana. Here, in the Harivamsa Purana, we come across a short teaching on the oneness of Shiva and Vishnu as taught by Markandeya. This teaching of the oneness of the central Deity of Shaivism and Vaishnavism is not unique to this Purana, and is an important teaching across the vast Puranic literature. While the Hindu religion of the Goddess has long been consolidated, the Hindu religion of God has historically struggled to consolidate. The early Puranic literature, therefore, introduces the concept we now call the Trimurti, which tries to bring together the three religions of God: Rudra-Shiva, Vasudeva-Vishnu and Prajapati-Brahmā as one. With the rising of later sectarian Puranas and blossoming of Tantric (Agamic) literature, the religions of Rudra-Shiva and Vasudeva-Vishnu subsume the religion of Prajapati-Brahmā to the point where the latter becomes a secondary Creator. Herein, we already observe that process occurring. Brahmā, Vishnu and Rudra are identified as one, but a few verses later, Narayana (Vishnu) and Maheshvara (Shiva) are also called the primary causes of Hiranyagarbha (Brahmā).
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Markendeya said:Shiva is identical with Vishnu and Vishnu is identical with Shiva. I do not see any difference. They are both auspicious, without beginning, middle or end, eternal and undecaying. Hear! I will describe that form identical with Hari and Hara.
He is Vishnu who is Rudra; and He is Rudra who is Brahmā. Brahmā, Vishnu and Rudra are one and of the same form. Three of them are great ascetics, Lords of half-female forms, self-sprung, Givers of boons and masters of the universe. As water is mixed with water when it is thrown into it so Vishnu becomes at one with Rudra, when He enters into Him. As fire becomes fire when it is mixed with it so Rudra becomes identical with Vishnu when He enters into Him. Rudra is identical with fire and Vishnu is identical with the moon. This universe, consisting of mobile and immobile creations, is identical with the Protector of what is mobile and immobile in this universe. And Maheshvara is their Destroyer. The Lord Narayana, identical with past, present and future, and Mahesvara are the primary causes of Hiranyagarbha and the great principles. They both are the Givers of the Vedas. They are the Creator and the Protector of the universe. They send down rain in the shape of Indra and spread rays in the shape of the sun. They blow in the shape of the wind and create all. Thus, O grandfather [Vaishampayana], I have described to you the great secret. The man, who daily reads or hears this account, attains to that most excellent region created by the power of Vishnu and Rudra.
With Brahmā, Hari and Hara are the Creator, Preserver and Destroyer of the universe. I will now chant their glories. Vishnu is adorable unto Rudra, and Rudra is adorable unto Vishnu. They are one, still they range in the world under two forms. Vishnu is not different from Shankara and Shankara is not different from Vishnu. Therefore, formerly Rudra and Upendra became one. Salutation unto Rudra and Krishna of one united body!
Salutation unto the three-eyed Deity, salutation unto the two-eyed Deity, salutation unto coppery-eyed and lotus-eyed.
Salutation unto the Holder of the earth, of the Holder of peacock-feathers and of Keyura. Salutation unto Him adorned with a garland of skulls, Him adorned with a garland of wild flowers, Him the holder of trident and him the Holder of discus, unto the standard of gold and Brahmā.
Salutation unto Him clad in skin and salutation unto Him clad in a yellow raiment. Salutation unto the Lord of Lakshmi and unto the Lord of Uma.
Salutation unto the Holder of trident and unto Him of mace. Salutation unto Him whose body is covered with ashes and unto Him who is dark-blue hued. Salutation unto Him who lives in the cremation ground and unto Him who lives in an ashrama. Salutation unto the Rider of a bull and unto the Rider of Garuda. Salutation unto Him who has more than one form, unto Him who has many forms, unto the Lord of destruction and unto Him who lies on the ocean. Salutation unto Him who has many forms and unto Him who is Bhairava.
Harivamsa Purana II:127:29-50
Om Namah Shivaya.