The object of this study is to interrogate the historical contingency of the common idiom "blood is thicker than water." To be more specific, the book asks: what is the role of Christianity in the development of this blood mythology lurking in everyday speech? To answer this question the book examines the concept of blood within Greco-Roman and early Christian contexts, investigating blood's significance beyond a mere biological substance. The analysis traces the evolution of blood's symbolic meaning in order to understand how it relates to conceptions of kinship, purity, and the divine. In the early chapters, the book surveys conceptions of blood and consanguinity in Greco-Roman thought, ranging from the mythologies of Homer and the histories of Livy to the medical descriptions of Galen and Soranus to the ritual descriptions of Jubilees and Ephrem the Syrian. By providing a general survey of Greco-Roman understandings of blood the book can show, in the remaining chapters, the way specific early Christians drew upon Greco-Roman notions of blood. By outlining how the singularity of the blood of Christ produces different understandings of blood in the writings of Irenaeus, Tertullian, and Origen, the author argues that Christians do not invent blood differentiation but they do intensify the symbolic power of blood to make a difference.