IN 1998, John Shelby Spong, then the reliably controversial Episcopal bishop of Newark, published a book entitled “Why Christianity Must Change or Die.” Spong was a uniquely radical figure — during his career, he dismissed almost every element of traditional Christian faith as so much superstition — but most recent leaders of the Episcopal Church have shared his premise. Thus their church has spent the last several decades changing and then changing some more, from a sedate pillar of the WASP establishment into one of the most self-consciously progressive Christian bodies in the United States.

As a result, today the Episcopal Church looks roughly how Roman Catholicism would look if Pope Benedict XVI suddenly adopted every reform ever urged on the Vatican by liberal pundits and theologians. It still has priests and bishops, altars and stained-glass windows. But it is flexible to the point of indifference on dogma, friendly to sexual liberation in almost every form, willing to blend Christianity with other faiths, and eager to downplay theology entirely in favor of secular political causes.

Yet instead of attracting a younger, more open-minded demographic with these changes, the Episcopal Church’s dying has proceeded apace. Last week, while the church’s House of Bishops was approving a rite to bless same-sex unions, Episcopalian church attendance figures for 2000-10 circulated in the religion blogosphere. They showed something between a decline and a collapse: In the last decade, average Sunday attendance dropped 23 percent, and not a single Episcopal diocese in the country saw churchgoing increase.

This decline is the latest chapter in a story dating to the 1960s. The trends unleashed in that era — not only the sexual revolution, but also consumerism and materialism, multiculturalism and relativism — threw all of American Christianity into crisis, and ushered in decades of debate over how to keep the nation’s churches relevant and vital.