In plain words
They cannot catch the air. If they close nose and mouth tightly, they choke. When the branches of trees rub against one another, fire is born. The man of those days thought it a wonder that though fire is in the wood, it cannot burn the wood. This is a work man cannot do. Births and deaths go on without end in the world. To this day no one has been able to discover their secret. They cannot discover what the cause of this birth and this death is. They cannot say what its inner meaning is. So men recognised that there are things in this world that men cannot do, whether by their intelligence, by their strength and ability, or by learning and authority. They came to surmise that behind all this there is some great power. They believed that this very power is a power beyond man. They set out to inquire into that ground without which humanity could not stand in this world, no living being could live in this world, and no tree could grow in this world. Their inquiries and researches can never be mere blind beliefs. Nor can they be mere delusion. By their own great powers of tapas alone they tried to know these things. They came to regard this as a kind of search for God. Even then there arose some who said He is, some who said He is not, and people too who kept to the middle path. Through such reasoning and inquiry, and through researches of many kinds, they carried on their search for divinity along many kinds of paths.
At first they came to believe that the sun is the chief Being on whom we rest, both for performing the daily rites of our daily lives and for recognising the secret of life. Without the sun there would be no days at all. Without the sun no living being could come into existence. All living beings, the trees as well as the beasts and birds, grow only through the rising of the sun. Lotuses and lilies too open only with the rising and setting of the sun. So they came to believe that the sun is the root cause of all creation, of its sustenance and of its decline. From that time they began their worship, holding that the sun is God in visible form. That is why Viśvāmitra, the maharṣi of that time, gave the sun himself the place of the chief deity in the Gāyatrī mantra.
They also came to believe then the truth that a power called divinity pervades the inner and the outer, form and nature, and what is seen and what is unseen.