Susan Jacoby's latest book, A Secular History of Conversion, is a comprehensive look at the reasons people have converted throughout history, beyond the spiritual - coercion, economic gain, intermarriage, political advantage, among others. 

She speaks with Terry Gross on Fresh Air here about her reasons for writing the book, and the fact that "more than half of Americans will change religion at least once in their adult lifetime."

A short excerpt from the chapter entitled "Heinrich Heine (1797-1856): Convictionless Conversion", pages 241-243:

Heine wrote, "From my way of thinking, you can well imagine that baptism is an indifferent affair. I do not regard it as important even symbolically, and I shall devote myself all the more to the emancipation of the unhappy members of our race. Still I hold it as a disgrace and a stain upon my honor that in order to obtain an office in Prussia - in beloved Prussia - I should allow myself to be baptized."  

Heine, devoted to Prussia and to the German language, which he felt was his home, failed to achieve the levels of employment he had hoped for after conversion, an unfortunate result for a man so angry at himself for undergoing baptism.