Located on the corner of Boylston and Arlington Streets, Arlington Street Church is where the sect of Christianity known as Unitarianism reached its full flower. (In fact, it was once known as the “Boston religion,” based on its association with the social and political elite of the time.)
Among other social causes, Unitarianism threw its weight early on behind the abolitionist movement, led by preachers Theodore Parker and William Ellery Channing, who has a statue dedicated to him across the street from the church. The sect, now known as Unitarian Universalism, still has its headquarters in Boston  on Beacon Hill , and was recently one of the strongest religious voices supporting gay marriage. (Arlington Street Church itself has for a long time had an openly lesbian minister.)
Inside Arlington Street Church, the highlight is a collection of Tiffany stained-glass windows. Installed over thirty years at the beginning of the 20th century, the windows are subtly breathtaking, with light from outside filtering through multiple layers of opalescent glass and skin tones so real they might as well be warm to the touch.