In the later sixteenth century, 'reason of state' was a vogue term in practical discourse, not a theory-backed concept. In order to cope with what they thought it designated, orthodox Catholic and Protestant thinkers had first to construct a coherent identity for it. In doing so, they also conflated it with 'Machiavellism' and the politiques. 'Reason of state' thereby acquired theorization and canonical authors. This essay seeks to show that defenders of Catholic religious and moral orthodoxy, notably Jesuit writers, did not find reason of state wholly repellent or intractable, but on the contrary largely domesticated, and appropriated, it.