Liberal Protestantism developed in the 19th century out of a need to adapt Christianity to a modern intellectual context. With the acceptance of Charles Darwin's theory of natural selection, some traditional Christian beliefs, such as parts of the Genesis creation narrative, became difficult to defend. Unable to ground faith exclusively in an appeal to scripture or the person of Jesus Christ, liberals, according to theologian and intellectual historian Alister McGrath, "sought to anchor that faith in common human experience, and interpret it in ways that made sense within the modern worldview." Beginning in Germany, liberal theology was influenced by several strands of thought, including the Enlightenment's high view of human reason and Pietism's emphasis on religious experience and interdenominational tolerance.
The sources of religious authority recognized by liberal Protestants differed from traditional Protestants. Traditional Protestants understood the Bible to be uniquely authoritative (sola scriptura); all doctrine, teaching and the church itself derive authority from it. A traditional Protestant could therefore affirm that "what Scripture says, God says." Liberals, however, seek to understand the Bible through modern biblical criticism, such as historical criticism, that began to be used in the late 1700s to ask if biblical accounts were based on older texts or whether the Gospels recorded the actual words of Jesus. The use of these methods of biblical interpretation led liberals to conclude that "none of the New Testament writings can be said to be apostolic in the sense in which it has been traditionally held to be so". This conclusion made sola scriptura untenable. In its place, liberals identified the historical Jesus as the "real canon of the Christian church".
The two groups also disagreed on the role of experience in confirming truth claims. Traditional Protestants believed scripture and revelation always confirmed human experience and reason. For liberal Protestants, there were two ultimate sources of religious authority: the Christian experience of God as revealed in Jesus Christ and universal human experience. In other words, only an appeal to common human reason and experience could confirm the truth claims of Christianity.
Liberals abandoned or reinterpreted traditional doctrines in light of recent knowledge. For example, the traditional doctrine of original sin was rejected for being derived from Augustine of Hippo, whose views on the New Testament were believed to have been distorted by his involvement with Manichaeism. Christology was also reinterpreted. Liberals stressed Christ's humanity, and his divinity became "an affirmation of Jesus exemplifying qualities which humanity as a whole could hope to emulate". Liberal Christians sought to elevate Jesus' humane teachings as a standard for a world civilization freed from cultic traditions and traces of "pagan" belief in the supernatural.
As a result, liberal Christians placed less emphasis on miraculous events associated with the life of Jesus than on his teachings. The effort to remove "superstitious" elements from Christian faith dates to intellectually reforming Renaissance Christians such as Erasmus (who compiled the first modern Greek New Testament) in the late 15th and early-to-mid 16th centuries, and, later, the natural-religion view of the Deists, which disavowed any revealed religion or interaction between the Creator and the creation, in the 17–18th centuries. The debate over whether a belief in miracles was mere superstition or essential to accepting the divinity of Christ constituted a crisis within the 19th-century church, for which theological compromises were sought. Many liberals prefer to read Jesus' miracles as metaphorical narratives for understanding the power of God. Not all theologians with liberal inclinations reject the possibility of miracles, but many reject the polemicism that denial or affirmation entails.
Nineteenth-century liberalism had an optimism about the future in which humanity would continue to achieve greater progress. This optimistic view of history was sometimes interpreted as building the kingdom of God in the world.
Reformed theologian Friedrich Schleiermacher is often considered the father of liberal Protestantism. In response to Romanticism's disillusionment with Enlightenment rationalism, Schleiermacher argued that God could only be experienced through feeling, not reason. In Schleiermacher's theology, religion is a feeling of absolute dependence on God. Humanity is conscious of its own sin and its need of redemption, which can only be accomplished by Jesus Christ. For Schleirmacher, faith is experienced within a faith community, never in isolation. This meant that theology always reflects a particular religious context, which has opened Schleirmacher to charges of relativism.
Albrecht Ritschl disagreed with Schleiermacher's emphasis on feeling. He thought that religious belief should be based on history, specifically the historical events of the New Testament. When studied as history without regard to miraculous events, Ritschl believed the New Testament affirmed Jesus' divine mission. He rejected doctrines such as the virgin birth of Jesus and the Trinity. The Christian life for Ritschl was devoted to ethical activity and development, so he understood doctrines to be value judgments rather than assertions of facts. Influenced by the philosophy of Immanuel Kant, Ritschl viewed "religion as the triumph of the spirit (or moral agent) over humanity’s natural origins and environment." Ritschl's ideas would be taken up by others, and Ritschlianism would remain an important theological school within German Protestantism until World War I. Prominent followers of Ritschl include Wilhelm Herrmann, Julius Kaftan and Adolf von Harnack.