The moral knowledge argument contends that our capacity to acquire genuine moral knowledge—our ability to know moral truths with justified confidence—requires the existence of God as the ultimate ground of moral reality and our epistemic access to it. Unlike arguments that focus merely on moral ontology (what makes moral facts exist) or moral motivation (why we should be moral), this argument specifically addresses moral epistemology: how we can know moral truths reliably. The argument typically proceeds by establishing that (1) we do possess moral knowledge, (2) naturalistic accounts cannot adequately explain how we acquire such knowledge given the apparent disconnect between physical facts and moral facts, and (3) theism provides the best explanation by positing God as both the source of moral truth and the guarantor of our cognitive faculties' reliability in moral domains.
The moral knowledge argument has roots in early modern philosophy, particularly in the works of René Descartes and Nicolas Malebranche, who worried about the reliability of our faculties without divine guarantee. Contemporary defenders include Robert Adams in Finite and Infinite Goods (1999), where he argues that excellence and moral knowledge require a theistic framework, Alvin Plantinga in Warranted Christian Belief (2000), who extends his evolutionary argument against naturalism to moral knowledge, and Mark Linville in "The Moral Argument" (2009), which appears in The Blackwell Companion to Natural Theology. Richard Swinburne in The Existence of God (2004) also incorporates moral knowledge considerations into his cumulative case. Paul Copan and Mark Linville's An Introduction to Biblical Ethics (2014) further develops the epistemological dimensions of moral arguments.
Critics raise several objections to the moral knowledge argument. Evolutionary debunking arguments, advanced by Sharon Street in "A Darwinian Dilemma for Realist Theories of Value" (2006) and Richard Joyce in The Evolution of Morality (2006), contend that evolution can explain our moral beliefs without assuming their truth, undermining claims to moral knowledge on any worldview. Defenders respond that evolutionary accounts explain at most our belief-forming mechanisms, not the truth or justification of moral beliefs, and that theism better explains the correlation between our moral faculties and moral truth. Another objection concerns moral disagreement: if God designed our faculties for moral knowledge, why such widespread disagreement? Proponents reply that disagreement stems from cognitive limitations, cultural influences, and sin's noetic effects, not from fundamental unreliability of properly functioning moral faculties.
The moral knowledge argument differs from other moral arguments in its specific epistemological focus. While the objective morality argument claims that moral facts require God's existence, it doesn't address how we know these facts. The Kantian moral argument moves from moral obligation to practical postulates about God, but doesn't emphasize knowledge acquisition. Divine command theory concerns what makes actions right or wrong (God's commands), rather than how we know moral truths. The moral realism argument defends the existence of mind-independent moral facts but may remain agnostic about their epistemological status.