What is the central idea of The Religion of Man?

The central idea of Rabindranath Tagore's collection of essays, The Religion of Man, is what he calls "the creative principle of unity." This is an evolutionary principle which governs both the physical and spiritual evolution of life of on earth. For Tagore, however, it is through spiritual evolution—understood to be the unique province of human beings—that life reaches its most subtle refinements. Thus, the religion of evolution is not primarily the religion of earth but rather the religion "of man," which is the process through which human beings manifest the principle of unity.
Tagore defines the creative principle of unity as:

a marvellous quality of complex inter-relationship maintaining a perfect co-ordination of functions.

This is, he claims, "the divine mystery of existence, that baffles all analysis." Of this principle, he claims that:

what is most important of all is the fact that man has . . . attained its realization in a more subtle body outside his physical system . . . His multicellular body is born and it dies; his multi-personal humanity is immortal In this ideal of unity he realizes the eternal in his life and the boundless in his love. The unity becomes not a mere subjective idea, but an energizing truth . . . the consciousness of this unity is spiritual, and our effort to be true to it is our religion.

What is notable in Tagore's conception of the creative principle of unity is its overt indebtedness to French philosopher Henri Bergson's principle of "creative evolution." Tagore effectively fuses the central tropes of Bergsonian philosophy and Indian philosophy to create his characteristically mystical yet self-consciously modern religion.
Also notable in his work is the use of personalist philosophy, with its emphasis on relationships and community. With its incorporation of evolution into mysticism, Tagore's thought may be profitably considered in light of twentieth-century attempts to contrive religiously pluralist, scientifically conscious new modes of awareness.