Christian nationalism is a backlash against the success of secularism and the growth of nonreligion. Its proponents go so far as to claim that secularism is diabolical. This rejection of secularism poses a significant challenge for secular systems, which aim to be inclusive of diverse religious ideas. This book explains the depth of this challenge while recounting the views of Christian nationalists, including Donald Trump. It shows how key figures in the American tradition—Adams, Jefferson, Madison, and others—promoted political secularism. It uses Christian theology and modern political philosophy to argue in defence of secularism and against Christian nationalism.