Sriman Nārāyaṇa is the first Āchārya

There are many reasons why the Lord is attributed to our first Āchārya. The following can be considered the most significant reasons:

a) The Lord taught the Vedas to Lord Brahmā.

b) The Lord revealed the Dwaya Mantram to a scared Mahālakshmī when Madhu-Kaithaba stole the Vedas.

c) Through the teachings propagated to Lord Brahmā, He made sure that man reaps the benefit of the Vedas.

d) The Lord graced the four sons of Lord Brahmā- Sanatkumāra, Sanaka, Sananda, and Sanatsujata, and through them, He propagated Hita.

e) He is present as the Antharyami (the one who governs from inside) of great souls like Devarishi Nārada, Parāshara, and Parankusha.

f) The Lord incarnated as Hamsa, Hayagriva, Nara-Nārāyaṇa, Sri Krishna, and propagated Tattva-Hita-Purushārtha.

g) He made a great soul, such as Bhīshma, give the world great teachings such as the Neeti Shāstra.

h) At the end of Dwapara Yugam, when the teachings of the Pancharatra Agamas were lost, the Lord reincarnated as Sankarshana to re-establish them.

i) To establish the Āchārya-Sishya relationship, He incarnated as Nara-Nārāyaṇa and taught Nara the Tirumantram to Nara at Badrinath.

j) As access to the Vedas became restricted, the Lord, through the Āzhwārs, gave the world the Nalayira Divya Prabandhaṅgaḷ or the Tamil Veda, which propagated the message of the Vedas in a local language.

k) When the Buddhists tried to preach their teachings, which went against the Vedas, the Lord, through Āchārya's teachings, re-established the Vedantic Philosophy.