In "Narrative of the Life of Frederick Douglass, an American Slave," religion is a central theme that Douglass explores to highlight the hypocrisy and moral corruption of slaveholders. Douglass distinguishes between what he terms "True Christianity," which embodies love and compassion, and "False Christianity," which slaveholders use to justify their cruelty. Through his narrative, Douglass argues that religion, when corrupted, becomes a tool for oppression rather than a source of moral guidance.

Douglass provides a vivid example of religious hypocrisy through his master, Captain Auld. After Auld's conversion at a Methodist camp meeting, Douglass initially hopes for kinder treatment. However, Auld becomes even more brutal, using religious doctrine to justify his actions. Douglass writes, "I have seen him tie up a lame young woman, and whip her with a heavy cowskin upon her naked shoulders, causing the warm red blood to drip; and, in justification of the bloody deed, he would quote this passage of Scripture—'He that knoweth his master’s will, and doeth it not, shall be beaten with many stripes.'" This illustrates how slaveholders manipulated religious texts to sanction their inhumanity.

In the appendix of his narrative, Douglass clarifies that his critique targets the "slaveholding religion" rather than Christianity itself. He states, "I mean strictly to apply to the slaveholding religion of this land, and with no possible reference to Christianity proper; for, between the Christianity of this land, and the Christianity of Christ, I recognize the widest possible difference." Douglass condemns the false faith of slaveholders as "the climax of all misnomers, the boldest of all frauds, and the grossest of all libels." He emphasizes that true Christian values are incompatible with the practice of slavery.

Douglass's narrative also includes a parody of a hymn to underscore the hypocrisy of slaveholding Christians. This parody contrasts the outward religious practices with the brutal reality of slavery, highlighting the disconnect between professed beliefs and actions. Douglass asserts that the religion of the South serves as "a covering for the most horrid crimes, a justifier of the most appalling barbarity, a sanctifier of the most hateful frauds." He criticizes clergymen who support slavery, arguing that their actions betray the core tenets of Christianity.

Through his powerful language and personal experiences, Douglass exposes the moral contradictions of slaveholders who claim religious piety. His narrative challenges readers to reconsider the relationship between religion and morality, urging a return to the true principles of Christianity that reject oppression and embrace compassion.