[Martin Luther's] last and greatest reform of all was in congregational song. In the Middle Ages the liturgy was almost entirely restricted to the celebrant and the choir. The congregation joined in a few responses in the vernacular. Luther so developed this element that he may be considered the father of congregational song. This was the point at which his doctrine of the priesthood of all believers received its most concrete realization. This was the point and the only point at which Lutheranism was thoroughly democratic. All the people sang. Portions of the liturgy were converted into hymns: the Creed and the Sanctus. The congregation sang not "I believe," but, "We believe in one God." The congregation sang how the prophet Isaiah saw the Lord high and lifted up and heard the seraphim intone Holy, Holy, Holy.
-from Bainton's Here I Stand: a Life of Martin Luther, (Abingdon Press; 1950) p. 344.