In the Article of the Summa theologica on the question “whether infidels can have dominion and rule over Christians” (II–ii, q. 10, a. 10), St. Thomas says:
Dominion and rule are based on human law; the distinction between Christian and infidel springs from divine law. But divine law, which is founded in grace, does not destroy human law, which stems from natural reason. Therefore, the distinction between Christian and infidel does not, considered in itself, destroy the dominion and rule of infidels over Christians.