Om bhur bhuvah svaha
Tat savitur varenyam
bhargo devasya dhimahi
dhiyo yo nah prachodayat
Tagore was born in a family which, at that time, was developing a monotheistic religion based upon the philosophy of the Upanishad. He was brought up in an atmosphere of freedom from the dominance of any creed that had its sanction in the definite authority of some scripture, or in the teaching of some organized body of worshippers. At young age he somehow remained aloof at first, uninfluenced by any religion what ever. He could not persuade himself to accept any religious teaching because people around him believed it to be true.
Tagore was in fact "inspired by the tropical sky with its suggestion of an uttermost Beyond" as he described it. "The wonder of the gathering clouds hanging heavy with the unshed rain, with the sudden sweep of storms arousing vehement gestures along the line of coconut trees, the fierce loneliness of the blazing summer noon, the silent sunrise behind the dewy veil of autumn morning, kept my mind with the intimacy of a pervasive companionship." To him it was like folllowing the path of his Vedic ancestors ...
Then the Gayatri mantra was given to him during the ceremony of Brahminhood - it produced a sense of serene exaltation in his mind, although the real meaning of the verse at that age still seemed vague...
O golden sun in your heavenly gleam,
enlighten our heart and fulfill our mind,
so that we, knowing our unity with the devine heart of the Universe,
may encounter the Path which leads us to the entrance,
the gate of completion, guided by the splendour or Your Light.
Gayatri Mantra sung by Shekhar Ravjiani - YouTube
As a grown-up he had no difficulty in realizing this Being as an infinite personality in whom the subject and object are pefectly reconciled.
To him it was evident that his religion was a Poet's religion, and neither that of an orthodox man of piety nor that of a theologican.
As he later put it: "The man who questions mine opinion has every right to distrust my vision and reject my testimony."
[Source: Rabindranath Tagore - My life in my words - Uma Das Gupta - Penguin Books 2006 - page 320]