Mr. Schogol -
I would like to respectfully disagree with your claim that Tagore was not a Hindu. In terms of Christianity, Unitarianism may not be fully Christian, but in Hindu terms (and in Indian culture), the Brahmo Samaj is considered a part of the Hindu samaj (community). Being monotheistic does not make the Brahmo Samaj not Hindu. In the same way, if you are interested to research, is the Arya Samaj, which is Hindu and actually tied to the formation of the Brahmo Samaj.
Unlike Christianity, Hinduism does not have sacrament, or strict rules that define who is a Hindu and who is not. It is not monolithic but within it are many hundreds of denominations that vary much widely in their beliefs than the denominations in Christianity. It is a much more pluralistic in its setup.
Thanks for your time.