Worldwide, the number of people who call themselves Jews is about 14 million. They may all call themselves Jews, but what they mean by that name varies widely. These self−described Jews range from the most Orthodox, who have submitted themselves entirely to the imagined dictates of an imaginary god, to those who practice various forms of Judaism that are so watered down that they scarcely qualify as a religion, to those who observe no part of Judaism at all other than the celebration of a festival that they may call Hanukkah but that is in reality merely a Judaized version of Christmas. In this short book, I focus on the United States, which until recently had the largest Jewish population in the world−−just under six million self−identified Jews. Although it was recently surpassed by Israel, America arguably still has the most politically, socially, and theologically influential Jewish population in the world. According to a survey conducted in 2013 by the respected Pew Research Center, of those almost six million American Jews, 22% "describe themselves as atheist, agnostic or having no particular religion[.]" In the case of the youngest adult American Jews, the so−called Millennial generation, "32% describe themselves as having no religion and identify as Jewish on the basis of ancestry, ethnicity or culture." This large group of Jews, which is a growing percentage of American Jewry, as the above Millennial number shows, is commonly referred to "secular Jews," although some of them prefer the label "atheist Jews." These are the people I want to discuss in this book. I contend that they are not Jews in any meaningful sense of the word. They may wish to call themselves Jews for a number of emotional reasons, but I call upon them to be intellectually honest and accept that they have ceased to be Jews. They are ex−Jews. The contrary argument is based on the idea encapsulated in the phrase "once a Jew, always a Jew." For the anti−Semite, this phrase is used as a slur. It refers to negative character traits supposedly possessed by all Jews. To Jews who think that there can be such a thing as a secular Jew, the phrase refers to some innate quality, entirely apart from religion, that distinguishes Jews from their non−Jewish neighbors. What is that innate quality? That's the crux of the issue. Let's go hunting for it.