In De Legibus, On the Laws, Cicero offers a single formulation that later thinkers return to more often than anything else he wrote on religion or ethics: true law is right reason in agreement with nature, spread through all people, constant and eternal, and no senate or people can release us from it. The claim grounds legal and moral obligation in a rational order built into nature itself rather than in the decree of any particular state, an idea Cicero inherited from Stoic philosophy and restated in Latin so influentially that it became the standard citation for the whole later natural law tradition, the position that a moral order exists independently of human law and is knowable by reason.
Thomas Aquinas's treatment of natural law in the Summa Theologiae, the most influential statement of the idea in Western thought, builds directly on this Ciceronian and Stoic foundation. The line runs further still: Hugo Grotius invoked a version of it to argue that natural law would hold even if one granted, for the sake of argument, that there is no God, and Martin Luther King's Letter from Birmingham Jail cites Aquinas directly on the distinction between a just and an unjust law, a distinction that traces back through Aquinas to Cicero's own sentence. Comparable positions exist outside the Western tradition entirely, in the Mu'tazili claim that good and evil are knowable by reason and in the Confucian appeal to the way of Heaven, but it is Cicero's Latin formulation specifically that became the citation Western natural law argument keeps returning to.