Comparisons of different religious beliefs and practices wasn’t new in the 19th century. At least since the age of exploration, Europeans had been compiling long and detailed treatises comparing Christian with “heathen” religion. What was new in the 19th century was the formation of a discipline of comparative religion.
In his massive study of Philology, James Turner recounts how scholarship crystallized into a discipline when the numbers reached a “critical mass,” and they reached a critical mass because of the influence of liberal theology: “The indispensable wild card was the rise in the English-speaking world of an increasingly liberal Protestant theology. It explains a surge of interest, right in the middle of the nineteenth century, in non-Christian religions sharing both Christianity’s transnational reach and its basis in normative texts. Radical American Unitarians such as Theodore Parker and equivalent British theists like Frances Power Cobbe (adulator of Parker) provide the limit case. Throwing off first their Calvinist heritage, then discarding the authority of the Bible, these extreme Unitarians came to believe that ‘pure’ Christianity consisted in the voice of God in each individual heart. Creeds and institutions were transient, love of God and of one’s fellow human beings permanent; and truth thus appeared in all religions, under varied incrustations of human error.25 Even few Unitarians went as far as Parker in denying to Christianity any authority beyond the moral high-mindedness of Jesus, but many liberal Protestants who believed Christianity superior nonetheless saw truth in lesser faiths.”
For instance, according to Thomas Wentworth Higginson, “’Our true religious life begins when we discover that there is an Inner Light, not infallible but invaluable, which lighteth every man that cometh into the world.’ But not every man professeth Christianity; ergo, Christianity could hardly monopolize the light: “we might say that under many forms there is but one ‘religion, whose essential creed is the Fatherhood of God, and the Brotherhood of Man,—disguised by corruptions, symbolized by mythologies, ennobled by virtues, degraded by vices, but still the same.’”
Turner summarizes, “Motives ranged from the moderate conviction that missionary success demanded understanding and respect for non-Christian religions, to the post-Christian notion that all faiths were at bottom the same. Religion did not animate every pioneering scholar of comparative religion. But changing Protestant attitudes thrust the practice into new prominence, gave it new urgency. ”