Why did Hugo Grotius argue that natural law applies regardless of religious belief?

Hugo Grotius argued that natural law applies regardless of religious belief because he sought to establish a universal framework to restrain and regulate war and violence across a divided Europe. Writing during the Thirty Years’ War, he observed a lack of restraint that he believed necessitated a legal foundation independent of specific religious doctrines, which were often at the heart of the era's conflicts.1

In his masterpiece, De Jure Belli ac Pacis, Grotius famously asserted that natural law would retain its validity even if one were to concede the "wicked" hypothesis that God does not exist or is unconcerned with human affairs. By grounding his theory in human nature and reason rather than theology, he aimed to create a binding common law among all nations. While he recognized that secular law alone might not be sufficient to persuade rulers to minimize bloodshed, his argument for the independence of natural law from religion was a vital, strategic step toward achieving his practical goal of mitigating the cruelty of war.12