""Satan subtracts and divides,"" reads one church marquee. ""God adds and multiplies."" Mary and David Compton offer more than 400 such marquee messages in Life Is Short, Pray Hard: More Roadside Church Signs Across America. Many of these bite-sized sermons were sent in by readers of the Comptons' previous compilation, Forbidden Fruit Creates Many Jams. Some of the messages are cute, some are poignant (and a few, admittedly, are a little tedious). Taken together, they are a slice of Americana, a testimony to the endurance of folk religion in a multimedia age.