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 an untenable position. In its place, liberals identified the historical Jesus as the "real canon of the Christian church". German theologian William Wrede wrote that "Like every other real science, New Testament Theology has its goal simply in itself, and is totally indifferent to all dogma and Systematic Theology". Theologian Hermann Gunkel affirmed that "the spirit of historical investigation has now taken the place of a traditional doctrine of inspiration". Episcopal bishop Shelby Spong declared that the literal interpretation of the Bible is heresy. 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. In general, liberal Christians are not concerned with the presence of biblical errors or contradictions. 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 traditionally pagan types of 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 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.