Rig Veda II.1 states that there is only one Brahman, though attributed different epithets like Indra, Vishnu, Rudra, etc., to denote different actions of the same Almighty. The same was reiterated in RV I.164.46:
एकं सद विप्रा बहुधा वदन्त्यग्निं
ekaṃ sad viprā bahudhā vadantyaghniṃ
Though it is One, inspired poets speak of it in many ways
The Puranas on the other hand eulogise Vishnu, Shiva, etc, by deification, and demean Indra, Agni, Aswins, etc, even though according to Vedas, Indra, Agni, Aswins, etc, were epithets much like Vishnu and Rudra.
Deification of epithets is anti-Hindu or anti-Sanatana Dharma.
So how did the Puranas like Vishnu Purana, Shiva Purana, etc, justify the deification of epithets, as mentioned above, and advocating sectarian movements like Vaishnavism, Shaivism, etc., which are contrary to Rig Vedic principles?