How did Hugo Grotius use Christian teachings in his theory about the laws of war?

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Oil portrait of Hugo de Groot, also known as Hugo Grotius, by the workshop of Michiel Janszoon van Mierevelt, showing him with a ruffled white collar and dark clothing. — Hugo Grotius Portrait of Hugo de Groot (Hugo Grotius), oil on panel by the workshop of Michiel Janszoon van Mierevelt, 1631; in the Rijksmuseum, Amsterdam.

Hugo Grotius integrated Christianity into his legal framework to address the widespread lack of restraint during the Thirty Years’ War. While he famously grounded his theory in natural law—arguing it could theoretically function independently of religion—he recognized that secular norms alone were often insufficient to curb the violence of political leaders.1

To achieve his practical goal of minimizing bloodshed, Grotius employed a multilayered approach to law:

Law of God: When secular arguments failed to persuade rulers, he invoked the "law of God," primarily derived from the Old Testament.1
Law of Love: He incorporated the "law of love" and similar ethical mandates from the New Testament to regulate conduct and discourage cruelty in war.1

By layering these religious norms over his secular legal structure, Grotius sought to establish a moral and practical check on violence, ensuring that only just causes—such as defense or the restitution of property—could justify the resort to conflict.1