According to my understanding, if we omit interpolations like "Purusha Sukata", the Rig Veda contains pure spiritual aspects of of that era. Apparently, those hymns from Rig Veda might be referring to Indra (Over a quarter of the 1,028 hymns of the Rigveda mention Indra, making him the most referred to deity than any other), Agni, Vishnu, etc, but to the seers, who heard from within, they might have gave deep SPIRITUAL INSIGHTS.
In Rig Veda, the Almighty God was described in different names like Indra, Agni, Vayu, Vishnu, etc, according to the form, the Almighty's power was perceived.
a) The God was described as Indra, when he was praised as the saviour.
b) Vishnu indicates the all pervasiveness of the God. In Rig Veda, it was described as Vishnu made space by taking 3 great strides, for Indra for wielding him his Vijra, for eliminating Vritra.
Similarly, with Agni, Vayu, etc.
c) The allegorical stories of Indra's slaying of Vritra, etc, have deep SPIRITUAL meaning, but understood by later day generations in too literal ways.
d) This resulted in evolving of ritualistic concepts like Yagna, sacrifices, etc, in Yajurvedic era.
This lead to converting allegorical stories of/epithets like Prajapati used in Rig Vedic era into deifying different Gods like Brahma, Indra, Vishnu, into fixed forms and stories like Varaha, attributing to Prajapati Brahma, etc, were woven.
स वराहः ततो भूत्वा प्रोज्जहार वसुंधराम् | असृजच् च जगत् सर्वम् सह
पुत्रैः कृत आत्मभिः || २-११०-४
"Thereafter, that Brahma, assuming the form of boar, caused the earth
to rise from water and with his sons of pure soul, created the entire
Ayodhya Kanda, Ramayana
e) This decline in the capacity of successive generations in comprehending the pure SPIRITUAL concepts and consequential development of ritualistic ways, deviated the people from the pure SPIRITUAL CONCEPTS, further deteriorating the system into fixed ritualistic society.
f) Even in Ramayana, importance was given to physical rituals and Sri Rama was described as expert in Yajurveda (यजुः वेद विनीतः - 35th Sarga 14th Sloka of Sundara Kanda).
It does not mean to say that Ramayana occurred at that point of time, but was composed in that ritualistic Yajurvedic period.
g) It was in that chaotic period, Gautama, the Buddha was born. The teachers of ritualistic generation could not offer proper answers to the Buddha. Hence, he discarded the ritualistic methods and found his own way of SELF REALISATION.
h) His methods may be different, but the ultimate goal is the same as the seers of Rig Vedic visualised. However, the successive followers of Buddha could not fully comprehend his ABSTRACT concepts, and they also got degenerated and split into groups.
i) Finally, Sri Sankara was forced to held arguments at length with various scholars of his time for bringing back many of them in to the fold of SANAATANA DHARMA and purging of Buddhism from India.
However, due to his shorter life span, he could not rejuvenate the Rig vedic concepts to original place. Perhaps, due to this vaccum created by his departure, the 6 methods of worship, writing of Puranas, etc, were taken place.
In SPIRITUAL concepts, irrespective of understanding of the scriptures, one should have been gifted with INTUITION, without which the real meaning of the SPIRITUAL CONCEPTS, can NEVER be understood.
1) INDRA, the Chief of Gods in Rig Vedic Period, was degraded in the later part of literature and was attributed with so many vices and avarice, viz., his misadventure with Ahalya, etc.
2) Prajapati appears late in the Vedic layer of texts, and the hymns that mention him provide different cosmological theories in different chapters. His role peaked in the Brahmanas layer of Vedic text, then declined to being a group of helpers in the creation process.
This prajapati concept was converted into Brahma in later stages and 6 Prajapatis were stated to have been created by Brahma. Brahma was temporarily elevated to the status of Creator of all in Ramayana, but later degraded and placed next to Vishnu and Shiva. Further, he was attributed with various vices, like his lust towards Parvati.
3) We do not find Shakti, Ganapati, etc, in Vedic Era. Even in Bhagvad Gita we do not find mention of Shakti, Ganapati.
4) In Puranas, Vishnu, Shiva, Shakti, etc , were elevated to the level of SUPREME GODS, and Indra was defamed, while attributing various vices.
5) Yama was not mentioned in the oldest part of Rig Veda, but had been mentioned in Mandala 1 and 10.
There was a clear shift, in my view, from Pure SPIRITUAL CONCEPTS of Rig Vedic era to ritualistic Yajurveda period. This shift was more pronounced in Puranas.