Cohen was the son of a cantor, or worship leader, at a Philadelphia synagogue. His father wasn't comfortable with him marrying someone who wasn't Jewish. At first, Cohen didn't want to push his faith on his fiancee, but Jane really loved Jewish rituals like lighting Shabbat candles and eating with family on Friday nights. She decided to convert, taking the name Sarah.

A few years later, Sarah got very sick. As friends and doctors gathered around her, assuming she was dying, she had a vision of Jesus. This was what real conversion felt like, she thought; it was so much deeper and more heartfelt than her earlier turn to Judaism. The Cohens tried to make it work, but they fought over keeping her faith a secret and how to raise their kids. Eventually, they split. One of their daughters went to live with Abraham as a Jew, while the other two followed their mother as Christians.

The thing is, Sarah Jane and Abraham Cohen first met in 1806. They separated in 1831 after one of their sons died of scarlet fever. While their story has a certain 19th-century flavor to it, the same thing might have happened to any couple currently on the dating scene. Perhaps it would be comforting to Sarah Jane and Abraham to know that their descendants are likely facing the same questions and having the same fights as they did, 200 years later.

Lincoln Mullen, an assistant professor of history at George Mason University, writes about the distinctively American fascination and struggle with conversion experiences in his new book, The Chance of Salvation. Throughout the country's history, he argues, people have been choosing their religion. This shows up even in today's demographic data: According to Pew Research Center, more than one-third of Americans currently identify with a religion that's different from the one they grew up with, and that number is much higher depending on how you count. It's particularly fascinating to watch this trend among the young people who are turning away from religious institutions in large numbers, as we wrote about in our "Choosing My Religion" series last spring.

Mullen and I spoke about the attraction--and myths--of conversion in America. Our conversation has been condensed and edited for clarity.

Emma Green: There's a myth out there about American religion which goes something like this: We have reached a crisis point in religious affiliation. Everyone is running away from traditional religious observance, and religion is going to die.

Your book significantly undermines that myth. You describe a period in the early 19th century when many Americans weren't very religious, followed by a period of religious revival, largely driven by aggressive proselytism. What have you learned about the myth of the so-called "nones"--people who aren't any religion in particular--through your study of conversion?

Lincoln Mullen: In many ways, the early 19th century was more like the period we're living in today than the 20th century. If you asked people to draw a chart of religious affiliation in history, I think most would say it's gone down constantly over time. It's simply not the case that everybody, or even the majority of people, were associated with a Christian denomination or other religion.

In the early 19th century, Christian missionary organizations were really quite worried about the lack of Christian affiliation they saw in society. There was this tremendous effort to reach people and convert them to Christianity. Over the course of the 19th century, through their missionary effort, more people became affiliated.

Green: We often think of conversion as being freely chosen. But the missionary zeal you describe complicates that. How did this concerted battle to save souls shape the landscape of conversion?

Mullen: This missionary effort I've been talking about creates an obligation to choose your religion. It works like this: Somebody has a religious affiliation, and then an encounter with a missionary or person of a different background makes them realize that there are other options out there. Those other options might not be appealing, but the presence of all those different options creates a sense that if you stay in the religion to which you're born, you're staying in it because you've made a choice to remain affiliated and reject other options.

All of this is unequally distributed. If you're a Christian growing up in a place with a lot of Christians, you're much less likely to be affected by the knowledge that there are other options available to you. For instance: If you're a Catholic in a Catholic enclave, then your religion is closely related to your ethnicity and neighborhood. But if, for instance, you're an American Jew, you're probably in a very small minority. You're almost certain to be approached by Christian missionaries. So there's a kind of asymmetric experience here.

"Religion is a kind of perpetual-motion machine. It brings destabilization as well."

Green: When does that cross over into coercion? You describe African American encounters with proselytization and coercion before and after the Civil War. Religious outreach has a much different valence for a person who is not free to make his or her own decisions. How do you think about that line?

Mullen: There's a double question when it comes to African American Protestant conversions. They share an awful lot with white Protestant conversions. But in the U.S., the color line is always stronger than Christian unity.

Sometimes, slave owners encouraged conversions to a bastardized Christianity which supported only obedience to masters and had none of the claims of liberation you would find elsewhere in the Bible. At other times, slave owners did everything they could to prevent enslaved African Americans from knowing about Christianity. There are many, many black conversion accounts from slaves who would go to revival meetings and then be whipped by their master. And it's well known that slaves sometimes had to hold their own meetings separate from white Christians in places called hush harbors. There's constraint on the freedom black Christians have to practice their religion.

Green: All of this underscores the meta-point of your book: "Religious choice" should have really big air quotes around it. "Choice" does not necessarily happen freely in a vacuum, as we like to imagine.

Mullen: Religious historians have often used the metaphor of a "free market" in religion without deeply examining what that metaphor means. It's a throwaway line. When I read conversion accounts from so many different kinds of people, they sounded a note of loss as much as gain.

That loss could be your family. If you left the religion of your birth, you were leaving behind your parents or perhaps your spouse. Sometimes people made choices that lost them social position. Other times it was just a sense of leaving behind a former self.

Religion is a kind of perpetual-motion machine. For many people, it brings stability, but it brings destabilization as well. While there is freedom in that system of American religion, I think it also comes at a tremendous cost.