Imagine a beautiful blond couple with 20 years of marriage and two kids under their collective belt. They live in a comfortable house in a Birmingham neighborhood they can almost afford. They have successful careers. They are Christians.

One day, they attend a wedding in upstate New York. A much-needed break from the kids? A second honeymoon? Nope. As they lie together in the hotel bed, she confesses to him that she's having an affair, and then he confesses to her that he is, too. The conversation is really too painful to have out loud, so they play a numbers game, holding up one finger for yes, two for no. Do I know him? Do you love her? Do you love him? All ones, all yeses.

If you want to read more, check out Dennis and Vicki Covington's book, "Cleaving: The Story of a Marriage." Their editor wanted the Covingtons to write a book about how, after many rough years of alcoholism and infidelity, they found God and turned a marriage that looked like disaster into something stable, solid, even idyllic. Instead, it's the story of how they found God and cheated on each other anyway.

Not because they hated each other. Not because they were any more estranged than any other married couple. Just because. Because they were fidgety. Because they were a little bored, and two interesting, attractive locals presented themselves.

I say none of this to pan the book. It's a wonderful, if horrifying, book, sentence after carefully crafted sentence. The Covingtons are elegant writers. They write honestly. And the book is devastating, precisely because they love each other and they love God and they cheated on each other anyway--are still cheating, for all I know. It's devastating because it makes clear just how easy having an affair can be.

My friend Tim and his wife, Karen, read "Cleaving," and they both had quibbles with the book. Tim's quibble was about the meaning of "Christian," and Karen's was about the meaning of "affair." Vicki Covington, Karen said, hadn't been having an affair because she hadn't been having sex with the man in question. She had written him tender letters. She had collapsed exhausted into his arms. She had fallen in love with him. But she hadn't, in Shakespeare's phrase, made the beast with two backs.

Dennis Covington, Tim said, wasn't a Christian--and probably his wife wasn't either. Their conversions must have been insincere, maybe even manufactured for the book; for if they truly had become born-again Christians--and so recently--they never would have been able to spit in God's eye by unzipping their pants just because two intriguing potential lovers happened along.

Most Christians, of course, would recognize that both Tim and Karen are wrong. Tim is wrong because Christians sin. Christians sometimes cheat on their spouses, just like sometimes we lie and horde and sadden God in 1,000 other ways. And Karen is wrong because adultery is about more--and sometimes less--than sex.

As I see it, when you stand before God and your friends and family in a white dress or a black tux and swear off other people until you die, you're promising to do more than refrain from hopping in the sack with another person. You're also promising that you won't make someone else the primary person in your life. If you meet another man and you tell him secrets about your marriage, confide your anxieties about your husband, whisper that you love him, and tell him over the phone that you long to touch his hand--then you've been unfaithful.

Nor does one have to be married to be unfaithful. True enough, cheating on your girlfriend doesn't involve breaking marriage vows, but it's infidelity nonetheless.

We Christians like to think of infidelity as someone else's problem--godless types who spend a lot of time at R-rated movies are prone to infidelity, but not those of us who are in a Personal Relationship With Christ. For us, fidelity is easy.

Of course, that's not true. Most of us know someone from church who's cheated on someone. Happily, Christian readers can find, of late, a lot of good, honest discussion about infidelity--discussion that acknowledges how tempting affairs are and that speaks frankly about how and why to avoid them.

In the most recent issue of Marriage Partnership, for example, Annette LaPlaca calls fidelity "A Hard Habit to Fake." Fidelity, she writes, "isn't the last-minute, should-I-or-shouldn't-I decision made in the clutches of temptation." It's a routine, and it takes practice. "A long-cultivated habit of fidelity is the best way to prevent the 'big' infidelity of adultery," LaPlaca says. "Every day, we need to form the habit that will keep us out of the wrong person's bed."

There are some practical things one can do to prevent infidelity. My latest temptation is a guy I work with named Sven, who has beautiful eyes and has been flirting with me lately. (He has a girlfriend, by the way. And he's a churchgoer.) I work with Sven, so I can't avoid him completely.

But I don't spend time alone with him. And I never let our conversations go beyond the most superficial topics--if he's my soul mate, I don't want to find out.