My own history with chastity is nothing to be proud of. I first had sex when I was 15, with a guy I met at summer camp. We dated for three months and had sex, but gradually our relationship dissolved—he went away to college, we wrote letters occasionally, but things fizzled out. A year later, I started college myself. And even though I was part of an observant Jewish community, I kept having sex. My freshman year, I dated a stunning man (he looked like an Armani model), and we had sex a few times. Then I began dating the man I now think of as "my college boyfriend," and we had sex too. None of this behavior was sanctioned by my Jewish community, so I kept it pretty quiet.

As I graduated from college and moved from New York to England for graduate school, I got pretty serious about Christianity. I was going to church regularly by then, praying to Jesus, thinking about him as I walked down the street, believing with a certainty that surprised me that he was who he said he was: God. I did some of the things you might expect from someone who believes that Jesus is God. I got baptized. I started spending inordinate numbers of hours hanging around with other Christians. I read the Gospels. I prayed the Psalms. I wore a small silver cross around my neck, proclaiming to passersby that I am part of this tribe whose allegiance is to Jesus.

But there were other things that you might expect a Christian to do, and I did not do them. I didn't forswear sex. I didn't tithe. I didn't especially enjoy going to church on Sunday mornings; in general, I had to grit my teeth, silence my alarm clock, and drag myself there.

I knew, dimly, that Christianity doesn't look kindly on premarital sex, but I couldn't have told you much about where Christian ...