Eliot was born on 26 September 1888 in St. Louis, Missouri, the youngest child of a teacher and a merchant. His childhood was spent on the banks of the Mississippi. This great river, which was to figure so powerfully in his Four Quartets, had first been under the domination of the French, and it is from the French that the town gets its name. However, in 1803, Napoleon, preparing for France's titanic final struggle with the English in their 600-year war, raised much-needed cash by selling France's holdings in the United States to Jefferson. This meant that St Louis became the economic staging post for the American republic's move to dominate all the land from the Mississippi to the Pacific Ocean. Indeed the settlers already moving West had been a major reason in Jefferson's decision to purchase the Western half of the Mississippi basin. From the middle of the century this movement became one of the greatest migrations of people that the world has known. If the British Empire was established under the nostrum that trade follows the flag, the polity founded by the Pilgrim Fathers placed trade under the tutelage of the Church. It was, therefore, not surprising that some of the best and the brightest of New England's sons should choose to follow their calling by preaching on this newly established trade route. A key figure in this development of St. Louis was Henry Greenleaf Eliot, a Unitarian minister.

The most radical theological wing of the Protestantism that had developed on the north-east coast of America in the eighteenth century was Unitarianism, which denied the doctrine of the Trinity, making Jesus an exemplary man rather than a god, and which abandoned classic Calvinist doctrines of predestination in favour of an effort to live a good life on earth. Unitarianism has ancient theological ancestors and can be traced back to Arius in the fourth and Arminius in the seventeenth centuries, but it gained particular force in America in the nineteenth century, when it could easily accommodate both the philological developments that made literal belief in the divinity of Jesus more difficult and the economic developments of a modern industrial society that made social improvement a pressing concern.