As families gather around buffet tables smothered with food on Thanksgiving, religious diet groups caution us God might not approve of that second piece of pie. Yes, that's right. The omnipresent world of wonder diets and slim-down regimes now has a foothold in the world of the omnipotent.

Faith-based weight loss groups have been a quietly growing presence for more than three decades. Organizations such as First Place 4 Health, a...