The difference between just taxation and legal plunder is an important question for Christian ethics, dating back at least to Augustine, who raises the question of the difference between just kingdoms and great robberies in the City of God. In this article, I provide an exposition of the thought of the nineteenth-century economist Frédéric Bastiat on this issue. I argue that Bastiat’s thought stands in the Christian tradition of normative natural law, which informs Augustine’s question. For Bastiat, two moral principles govern taxation: equivalence of value for value as the definition of justice in exchange and legitimate defense as the sole justification for force. He concludes that the state’s moral authority to effect forced exchanges of tax payments for public services is derived from its unique role as guardian of the public peace and administrator of the public domain. Taxes levied for purposes beyond this limited role constitute legal plunder.