I've read somewhere that in the vedic period Vishnu was not the main deity of worship. It was Indra, Mitra and other deities. Later in the puranic times Vishnu and Shiva rose to prominence. What made the people shift from the vedic to the puranic Gods? Is it because people realized that they were worshipping inferior gods before?
Hi Greg. Why you are always asking very deep questions? Are you cursed as like me?
This questions I was also asking few months before, and the questions are answered - and I will express it here.
When I started to study Srimad Bhagavatham, these deep questions about Gods,etc., revealed some insight.
All the Gods are sons of Aditi and Kashyap. Among them, Indra was first. Even, Vishnu and Rudra are born after Indra,Varuna,etc.,
On those times, Vishnu and Rudra are normal demi-gods like Indra,etc., - were Vishnu takes care of Maintanence and Rudra takes care of duty of destruction.
So, for Karmic desires, in all 4 classes, Indra and other gods are prayed for their desires shall be fulfilled. And so, Vishnu and Rudra gods are neglected - as they are not gods who fulfil Karmic desires, but maintains and destructs things.
Then came Vyasa. He created a strong belief on those two and Sakthi and etc., by getting blessings from them and wrote Puranas.
Why these Puranas are made is to, understand the Truth - to stop desiring for Karmic things.
To stop desire, devotion is the easiest way. He got blessings from those Gods like Vishnu and Rudra and Goddess Durga/Sakthi, etc., and created stories method and give the moral in end/middle of stories, like how to pray or be devoted to them and how to live desirelessly, etc.,. If one be totally devoted them, then easily saalokya,etc., moksha can be achieved. They are all said in different upanishads how to pray/devoted to them by manthras (like
Om Soum Saravana Bhava Sreem Hreem Kleem Kloum Soum Namaha), and Vyasa inserted them inbetween or end of stories, for deep devotion. Also, Mahabharatham is a great moral story to understand the working of world and people and etc.,
The Puranas make a strong belief about their devoted god, so that even more strong devotion happens by leaving the karmic desires aside.
In the father of Tamil Siddha - Agastya song, it is called as "Vyasa's Play". Atleast in devotion to god, they might attain Moksha, if one is not interested to know things/Sankhya.
But, the gods like Vishnu/Rudra, are born same like Indra,etc., but free from all karmas and do Karma for Karma sake. Do the duty for duty's sake without any desire on good/bad results. Indra and some other gods - if become very ahankar, in fear of Rudra and his pisachas and demons, reach Brahma and Vishnu for help to make this set right. Indra and other gods made their higher god as "Vishnu", whereas demons and others made their higher god as "Rudra" - according to their nature of Sattvic,Rajasic and Tamasic.
If one is cursed to know things, like Buddha/Sages/etc., then those all Puranas and Vedas are just a creation out of "belief" and make things right by creating a procedure for Karmic desires, and those "beliefs" are bought into Life. Whatever one beliefs strongly and true to their heart, comes into life, but it's permanence is the thing questionable. there is no need of putting those vedas away - like Buddha did, but to see it's Karmic limitations and stop those desires inside us, and do those if someone needs it, for duty's sake, like God Vishnu does. Compassion/ Divine Love means to not give the right thing giver sees, but to give things whatever the seeker seeks.
The devotion/Bhakti, is all a creation (after the start of Kali Yuga - as said in Puranas), by giving the power to those 'beliefs' and bought into life. Before that, these Puranas were not there, and just "Karmic Vedas and Upanishads". But those beliefs are so strong, one cannot pierce it and kill those away, as it is blessed by those gods and bought into life by Veda Vyasa.
And, to the knowledge, those gods from Indra to end, even Kashyap and Aditi and Diti and all sages and their wifes, including the Sanat kumaras - are all born out of "beliefs/wants/thoughts". They are not real, but came to be appear real, like us, out of "thoughts" in the primordial Egg/seed.