This chapter introduces Christian nationalism. It is defined as a conceptual framework—a collection of myths, traditions, symbols, narratives, and value systems—that idealizes and advocates a fusion of Christianity with American civic life. The chapter also introduces the authors’ measurement of this concept and compares it to other possible measurement strategies. Finally, the chapter highlights three main arguments for this book. First, it argues that understanding Christian nationalism, its content, and its consequences is essential for understanding much of the polarization in American popular discourse. Second, the authors contend it is not synonymous with, reducible to, or a byproduct of personal religiosity, political views, racism, or authoritarian tendencies, among others. Finally, the claim is put forward that Christian nationalism is not “Christianity” or even “religion” properly speaking. The authors show that Christian nationalism influences Americans’ opinions and behaviors in the exact opposite direction that traditional religious commitment does.