This book asks why tax policy is both attracted to and repelled by the idea of justice. Accepting the invitation of economist Henry Simons to acknowledge that tax justice is a theological concept, the work explores theological doctrines of taxation to answer the presenting question. The overall message of the book is that taxation is an instrument of justice, but only when taxes take into account multiple goods in society: the requirements of the government, the property rights of society’s members, and the material needs of the poor. It is argued that this answer to the presenting question is a theological and ethical answer in that it derives from the insistence of Christian thinkers that tax policy take into account material human need (necessitas). Without the necessitas component of the tax balance, tax systems end up honoring only one of the three components of the tax equation and cease to reflect a coherent idea of justice. The book will be of interest to academics and researchers working in the areas of tax law, economics, theology, and history.