Lutheranism, branch of Christianity that traces its interpretation of the Christian religion to the teachings of Martin Luther and the 16th-century movements that issued from his reforms. Along with Anglicanism, the Reformed and Presbyterian (Calvinist) churches, Methodism, and the Baptist churches, Lutheranism is one of the five major branches of Protestantism. Unlike the Roman Catholic Church, however, Lutheranism is not a single entity. It is organized in autonomous regional or national churches, such as the Church of Sweden or the Evangelical Lutheran Church of Mecklenburg, Germany. Globally, there are some 150 such Lutheran church bodies; 148 of these are loosely joined in the Lutheran World Federation, which was established in 1947. Around the first quarter of the 21st century, there were more than 77 million Lutherans worldwide, making Lutheranism the second largest Protestant denomination, after the Baptist churches.

The term Lutheran, which appeared as early as 1519, was coined by Luther’s opponents. The self-designation of Luther’s followers was “evangelical”—that is, centred on the Gospel. After the Diet of Speyer in 1529, when German rulers sympathetic to Luther’s cause voiced a protest against the diet’s Catholic majority, which had overturned a decree of 1526, Luther’s followers came to be known as Protestants. However, because both evangelical and Protestant proved to be overly broad designations (before long they also included the Reformed churches), eventually the name Evangelical Lutheran became standard. Another name occasionally used is Churches of the Augsburg Confession, which recalls the Lutheran statement of faith presented to the German emperor at the Diet of Augsburg in 1530. In the United States several nomenclatures have been used, all of which, with the exception of the Evangelical Catholic Church, include the term Lutheran in their titles (e.g., the Lutheran Church–Missouri Synod).

In the 16th century, Lutheranism became formally established in various principalities by being declared the official religion of the region by the relevant governmental authority. As early as the 1520s German principalities and cities adopted Lutheranism, and they were later followed by Sweden and the other Scandinavian countries. Later, Lutheran notions found their way to Hungary and Transylvania. Lutheranism arrived in North America in the middle of the 17th century in the areas of present-day Delaware and southern Pennsylvania. In the 18th century and increasingly in the 19th, European and North American Lutherans undertook missions throughout the globe, leading to the establishment of indigenous Lutheran churches in many countries. Beginning in the 20th century, ecumenical initiatives affected both Lutheranism and its relation to other Christian faiths.

Theologically, Lutheranism embraces the standard affirmations of classic Protestantism—the repudiation of papal and ecclesiastical authority in favour of the Bible (sola Scriptura), the rejection of five of the traditional seven sacraments affirmed by the Catholic church, and the insistence that human reconciliation with God is effected solely by divine grace (sola gratia), which is appropriated solely by faith (sola fide), in contrast to the notion of a convergence of human effort and divine grace in the process of salvation.