In medieval times, gluttony was not associated with obesity (in fact the glutton was usually depicted as thin, owing to indigestion). But today we think of the glutton as fat — and the fat as gluttonous, although it's impossible to know whether gluttony was to blame.

"Now that gluttony has become an affront to prevailing standards of beauty and health rather than an offense against God," Prose writes, "the wages of sin have changed and now involve a version of hell on Earth: the pity, contempt and distaste of one's fellow mortals."

Lynne Gerber is getting a doctorate at the University of California at Berkeley's Union Theological Seminary and is writing her dissertation on Christian evangelical weight loss groups. They walk a fine line when it comes to obesity and sin, she reports. Most of the groups "think of gluttony as a sin, as disobedience to God, but they don't want to go so far as to say. 'If you're fat, you're sinful.'"

Americans eat 25 percent more calories than they need, "so there is a kind of excess behavior in the culture," says professor Lloyd H. Steffen, chaplain and professor of religion studies at Lehigh University. "A glutton consumes and doesn't pay attention to the bigger picture" — in this case the effect of overeating on pollution and land use. If we buy gas-guzzling cars, we're gluttons, too, he says.

Which brings us to the Rev. Daniel Webster's prayer. The Rev. Webster, who used to be spokesman for the Episcopal Diocese of Utah, is now spokesman for the National Council of Churches in New York City.

"This Lent, I shall pray that our country be healed of the sin of gluttony," the Rev. Webster wrote in a recent e-mail. "With 5 percent of the world's population, we consume 26 percent of the planet's natural resources. We, collectively, as a nation commit the sin of gluttony each time we leave on a light bulb in a room we are not using. Each time a carmaker offers a vehicle that gets 15 mpg and people buy it, our society commits the sin of gluttony."

If, in the end, we are uncomfortable thinking of gluttony as a sin, we might want to consider the ancient Greek take on it all. Gluttony in its classic formulation, says Steffen, "is not so much a religious term as a moral one" — an offense against the moral order. For the ancient Greeks, the moral life was the road to happiness. "And the way to do that was moderation."

So, how do you know when you've crossed the line? How do you know, in your own life — even if you aren't downing 13 pounds of spaghetti at the Alka-Seltzer U.S. Open of Competitive Eating — if you've become a glutton?