The Brahman of the Upanishads is equated with the personal God of religion in Vaishnavism, Lord Vishnu. The identification of Brahman with Narayana is established on the basis of the Upanishadic texts by adopting the principle of interpretation laid down by the Mimamsakas. According to them, when several terms are used in the same context in a passage, the words bearing the general meaning should bear the meaning of the specific word. The same logic is adopted in the matter of determining the meaning of the terms such as Sat, Brahman, Atman and Narayana. All these four terms are used in the Upanishadic passages in the same context of explaining the causation of the universe by Brahman at the time of creation. Thus states the Chandogya Upanishad, "This was in the beginning Sat only". The Aitareya Upanishad says, "All this was Atman only in the beginning." The Brhadaranyaka Upanishad points out, "All this was Brahman only in the beginning."
Thus it is seen that three different terms- Sat, Atman and Brahman are used. Since several entities cannot be the cause of the universe, it is obvious that only one particular entity can be the sole cause of the universe. Among these which one could be the cause? The word Sat is too general a term and may mean anything that exists. The term Atman is a little more specific but it may mean both jivatman and Paramatman. The word Brahman is relatively even more specific but it is applicable to more than one entity such as jiva and prakriti. Another Upanishadic passage, speaking of the creation of the universe, mentions Narayana as the cause of the universe. Thus, the Mahopanishad says, "Only Narayana existed (in the beginning)." The word Narayana is used in this passage in place of the terms Sat, Atman and Brahman mentioned in the other passages as the cause of the universe. According to the grammatical rule formulated by Panini, the term Narayana is treated as a specific proper name (samjiid pada), and is applicable to one specific Being only but not to any other entity as other general terms do. Ramanuja, therefore, concludes on the basis of the principle of interpretation explained above that Brahman, the cause of the universe, is the same as Narayana or God.
Further, the Taittiriya Narayana Upanishad (which is part of Taittiriya Aranyaka) emphatically asserts that Narayana is Para Brahma, Narayana is Para tattva and Narayana Paramatma. The Subala Upanisad describes Narayana as antaratma, the inner controller of all beings in the universe. Only that which creates the universe becomes Antaryai as is evident from the Upanishadic statement which says that after creating it, the same Brahman entered into it.
Narayana is not a mere name of the God of a particular cult. It is a term which connotes all the essential characteristics of the concept of the ultimate Reality of philosophy. According to the etymological meaning of the word, Narayana is one who is the ground of the entire universe of cit and acit. Nara means the universe of sentient and non-sentient beings. Ayana means one who is the ground for it (naranam ayanam). It also means one who is immanent in all (narah ayanarh yasya sah). That is, all that exists in the universe has for its ground Narayana. As the terms Vishnu and Vasudeva bear the same etymological meaning as that of Narayana, Brahman is also equated with these two names.
The important point to be noted in this context is that the ultimate Reality of metaphysics will have to be conceived as a Supreme Personal Being (Purusottama). It is philosophically justified because any other concept of Reality as a transcendental undifferentiated Being cannot have any causal relation with the universe. The acceptance of such a theory bridges the gulf between religion and philosophy because the personal God of a religion is not, and cannot be basically distinct from the ultimate Reality of philosophy.