NONFICTION : THE DEVIL AND DR. CHURCH by F. Forrester Church (Harper & Row: $12.95).

Does the devil exist? Sure enough, says Unitarian Pastor F. Forrester Church. And how can he be so certain? It takes one to know one, he confesses without flinching.

The reason many of the rest of us remain obstinately unconvinced is that we’re determined to look for the Evil One in all the wrong places. This, of course, is the scoundrel’s No. 1 ruse. But seeing through villainous deceits and virtuous disguises alike, the insightful Church concludes that the devil’s whereabouts can be reliably fixed “precisely where one least expects to find her.” Best spot to observe the androgynous creature: In the bathroom mirror.

Understandably, we’d rather look elsewhere. At liberals, say, if we’re conservatives; or the other way around. At atheists if we’re not; at fundamentalists if we are.

All such finger-pointing, blame-fixing and smug moral posturing suits the devil just fine, says Church. To distract ourselves from our daily complicity in sin is just what the charlatan has in mind. “The devil’s specialty is not lust, sloth, or gluttony . . . (it) is evil disguised as good.”

Exposing the devil’s hideout and tactics is not all author Church has in mind, though that’s an essential first step. The healing follow-up he prescribes is to resume the exercise of such biblical precepts as self-emptying, forgiveness, love for the enemy, love for the neighbor as oneself--neglected virtues against which the nastiest devilment cannot prevail. Do it even for pragmatic reasons, if that’s all you’re up to; you might help save the planet. “Nuclear disarmament and the love commandment are almost interchangeable.”

A trifle long for a sermon (aren’t most of them?), the 88 pages of “The Devil and Dr. Church” are just about right for an engaging, hope-filled invitation to self-examination and, just possibly, moral reformation.