Brahma, Vishnu and Shiva have differing relative importance in different Puranas, although they are always manifestations of the Supreme god.
There are eighteen main Puranas also known as Mahapuranas. Each of these Puranas considers the three aspects of the Trinity, Brahma, Vishnu and Shiva, as manifestations of the One Supreme God. However the Puranas do not consider these three manifestations as equal. Depending on the Purana one of the three is considered “more equal” than the others. In six of the Puranas Vishnu plays a slightly major role, in six of them Brahma is the more important one and in six it is Shiva who is more revered. This way each devotee can consider his favorite to be the leader of the Trinity and at the same time equality can be maintained.
Generally the One Supreme God is unnamed and formless. But in the Bhagavata Purana and some others Krishna is described as the Supreme God. Brahma, Vishnu and Shiva are described in these Puranas as manifestations of Krishna and function as sort of divisional heads, with Krishna being the CEO. According to these Puranas Krishna is not the eighth avatar of Vishnu, but Krishna, the Supreme God, himself descends to the earth to carry out the divine mission. Matters have come to such a head that the CEO has to take direct control of the activities.
Each of the Puranas had been initially narrated by a specific personage to a specific character for a specific purpose. Ages later Lomaharshana recited the Puranas again to another audience. It is usually with the second recital that the Purana begins as has been seen in the article 'Introduction to Bhagavata Purana'.
The Vayu Purana was first narrated by the demi-God of wind, Vayu. The sages had completed a 12-year yagna in the forest of Naimisharanya in the reign of King Pururava. They then requested Vayu to answer some questions they had and discourse that Vayu gave the sages is assembled in the Vayu Purana. Lomaharshana later narrated the Vayu Purana in the reign of King Adhisima Krishna, who was a descendant of Parikshit.
The Narada Purana was initially recited by Narada to Sanatkumar and his brothers Sanaka, Sanandana and Sanatana. These were the four sons of Brahma, produced through his mental power and were eternally in their youth. The four brothers were once bathing in the river Sita, which flows by Brahma’s city on mount Meru, when Narada passed by. The brothers stopped him and requested that he share his wisdom with them. These discourses constitute the Narada Purana. It is said that a person who listens to the Narada Purana with repentance in his heart is freed from the burden of all past sins.
The Brahmavaivarta Purana was narrated by Brahma to Dharma, the demi-God of Justice in Pushkar. The Brahmavaivarta Purana provides the essence of all other Puranas. It is said to fulfill the desires of those who hear it. It is this Purana that first narrated the tales of the romance of Krishna and Radha.
The Matsya Purana was narrated by Vishnu to Manu during his descent to earth in his first avatar, known as Matsya avatar. While Vishnu, in the manifestation of a huge fish was safely pulling the boat carrying Manu and the sages, he passed onto Manu the wisdom and information to repopulate the world after the deluge, which would destroy all life except what was on the boat.
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