What we have been saying would have a degree of validity even if we should concede that which cannot be conceded without the utmost wickedness, that there is no God, or that the affairs of men are of no concern to Him.According to many scholars of the past two centuries, it is Grotius we have to thank for freeing public international law from the bounds of Christianity and Christian theology. Of course, what we have in its place is little more than moral intuitionism writ large. For my thoughts on this you can read Looking for Bedrock: Accounting for Human Rights in Classical Liberalism, Modern Secularism, and the Christian Tradition.
I've always found this account of Grotius's role to be overly simple. After all, how could a man who also wrote one of the most significant accounts of the theological meaning of Christ's atonement have intended to insulate God from law?
Indeed, he did not. For a clear and cogent account of the distinctively Christian anthropological underpinnings of Grotius's theory of international law read Janne Nijman's Grotius' Imago Dei Anthropology: Grounding Ius Naturae et Gentium. You can download it here.
A brief quote states Nijman's thesis:
For Grotius and his contemporaries – whether scholastic or humanist scholars - the imago Dei doctrine provided the inevitable starting point for thinking about humanity and human nature. [T]he imago Dei doctrine played a significant role in the background of many theological and moral philosophical controversies at the time of the Reformation. ... Section 2 relates the concept of the imago Dei briefly to these early seventeenth-century theological and political debates in the Dutch Republic. ... This chapter claims that the Arminian imago Dei anthropology is foundational to Grotius’ theory of the law of nature and nations in (at least) three ways ...A good work which goes a long way toward deflating the pretensions of the secularization thesis.