Mark Twain is one of the most fascinating figures in American history. His literary works have intrigued, illuminated, inspired, and irritated millions from the late 1860s to the present. Twain was arguably America’s greatest writer from 1870 to 1910. In an era of mostly lackluster presidents and before the advent of movie, radio, television, and sports stars, Twain was probably the most popular person in America during the 1890s and competed with only Theodore Roosevelt for the title in the 1900s; his celebrity status exceeded that of European kings. Twain’s varied experiences as a journeyman printer, riverboat pilot, prospector, journalist, novelist, humorist, businessman, and world traveler, combined with his incredible imagination and astonishing creativity, enabled him to devise some of American literature’s most memorable characters and engaging stories. Twain was mesmerized, perplexed, frustrated, infuriated, and inspired by Christianity. He strove to understand, critique, and promote various theological ideas and insights. Twain’s religious perspective was complex, inconsistent, and sometimes even contradictory and constantly changed. While many scholars have ignored Twain’s strong focus on religious matters, others disagree sharply about his religious views, with most labeling him a secularist, an agnostic, or an atheist. The evidence indicates, however, that throughout his life he engaged in a lover’s quarrel with God. Twain was an entertainer, a satirist, novelist, and reformer, but he also functioned as a preacher, prophet, and social philosopher. He tackled universal themes with penetrating insight and wit including the character of God, human nature, sin, providence, corruption, greed, hypocrisy, poverty, racism, and imperialism. Moreover, Twain’s life provides a window into the principal trends and developments in American religion from 1865 to 1910.