As a general term in theological use, assurance refers to a believer's confidence in God, God's response to prayer, and the hope of eternal salvation. In Protestant Christian doctrine, the term "assurance", also known as the Witness of the Spirit, affirms that the inner witness of the Holy Spirit allows the Christian disciple to know that he or she is justified. Based on the writings of St. Augustine of Hippo, assurance was historically a very important doctrine in Lutheranism and Calvinism, and remains a distinguishing doctrine of Methodism and Quakerism, although there are differences among these Christian traditions. Hymns that celebrate the witness of the Holy Spirit, such as "Blessed Assurance" are sung in Christian liturgies to celebrate the belief in assurance.