The fundamental teaching of the Upanishads is to discover unity behind plurality. Unity is the Reality, and identification with this unity is to be achieved. The ultimate unity is variously called: Deva, Purusha, Akshara, Atman, and Brahman. The Samhitas and Brahmanas admit plurality of gods in their prayers and ritual practices. But, at the same time, the unity of gods did not escape the attention of the seers.
Largely similar — and sometimes even the same — epithets are
often ascribed to various gods, which implies they were regarded as essentially
one behind their apparent plurality. A god is often identified with other gods.
Gritsamada addresses Agni as being identical with Indra, Vishnu, Brihaspati,
Brahma, Mitra, Varuna, Aryaman, Tvashtri, Rudra, Maruts, Pushan, Savitri, and
Bhaga, among others.
Similarly, Aditi is everything; Aditi is the heaven, Aditi
the skies, Aditi is the mother, the father, the son’ (1.89.10). The obvious
conclusion is that the ‘one God’ is praised in different names and forms.
The one God has been explicitly proclaimed in several
sections of the Vedic Samhitas as well as in the Upanishads: ‘Eko devaḥ; the
one Deva’ (Atharva Veda, 3.13.4, 10.2.14); ‘Ekaḥ suparnaḥ; the one Bird’ (10.11
4.4); ‘Eko devah sarvabhūteshu gudhah; the one Deva hidden in all beings’ (Shvetashvatara
Upanishad, 6.11.); ‘Eko hi rudro; Rudra is but one’ (3.2). The one God has
become everything, as in the following expression of the Vedic seer:
One is Agni kindled in many a place, one the Sun shining
over all; One is Ushas illumining all this; the One has become all. (Rig Veda, 8.58.2.)
The same idea is present in the Upanishads:
There is one Supreme Ruler, the innermost Self of all beings, who makes his one form manifold. Eternal happiness belongs to the wise
who perceive him within themselves, not to others. (Katha Upanishad, 2.2.12)
Unity with the object of worship was achieved by some seers.
In his Nirukta, Yaska mentions a threefold classification of mantras: where the
object of praise is indirectly perceived, directly perceived, and within
oneself, respectively. In the adhyatmika hymns the seers identify themselves
with the Divine, the object of praise. The ‘Devi Sukta’ is a very famous example.
In another sukta, Brihaddiva Atharvana praises Indra, and concludes by
announcing himself to be Indra.
Vamadeva is famous as the seer of the fourth mandala of the
Rig Veda. Vamadeva claims to have been Manu, Surya, Kakshivat, Kutsa, and
Ushanas: (4.26.1); in other words, he identifies himself with all of existence —
and this identification is the goal of Vedanta: ‘Aham Brahmasmi, I am Brahman’;
‘Sarvam khalv-idam brahma, all this is indeed Brahman’. In the Brihadaranyaka
Upanishad (1.4.10) Vamadeva is presented as a knower of Brahman: the sage
Vamadeva, while realizing this (self) as That (Brahman), knew “I was Manu, and
the sun”.’ In the Aitareya Upanishad (2.1.5) too, in the context of tritiya
janma, third birth, Vamadeva announces: ‘Even while lying in the womb, I came
to know of the birth of all the gods. A hundred iron citadels held me down.
Then, like a hawk, I forced my way through by dint of the knowledge of the Self.’