Jungmann, Josef A., S.J. "Chapter One: Introduction: History, the Present and the Future." The Early Liturgy to the Time of Gregory the Great. (translated by Francis A. Brunner, C.S.S. R., Notre Dame, Indiana: University of Notre Dame Press, 1959, pp. 1-8.
Jungmann considers that an evalution of history is necessary to develop a valid evaluation of the present or to mark projections of the future (Jungmann 1959, 1). The study of liturgy is very important in this regard, not the least because Christians are still engaged with liturgy on a day to day basis. Finding the roots of modern practices can help us understand the overall structure of the Christian life (Jungmann 1959, 2).
The study of history of liturgy was sparked by the events of the Reformation, at which time apologists sought out sources of more current practices (Jungmann 1959, 3). However, Jungmann notes we have somewhat defective information from early Christianity. Not much was written about the actual liturgy, presumably because it was known through practice. However, Jungmann sees the skepticism of the 19th century as impetus for additional study (Jungmann 1959, 4). The documents discuvered and analyzed at the end of the 19th century and the start of the 20th century serve to connect ancient and modern worship.
Jungmann considers the most important sources of information to be in Justin's Apology, chapters 65-67, in the Apostolic Constitutions from the fourth century, from the Mystagogic Catecheses of Jerusalem, often appearing along with catecheses of Cyril of Jerusalem (Jungmann 1959, 5), and Ambrose's De Sacramentis. These sources were known before the 19th century. In more recent discoveries, Jungmann finds the Didache, Hippolytus' Tradition Apostolica, Serapion's Euchologica, including a complete text of a Mass, an anonymous work called the Peregrinatio Aeteriae, which describes church services from about the year 400, and Theodore of Mopsuestia's Catecheses (Jungmann 1959, 6, along with a fifth century Testamentum Domini Nostri Jesu Christi (Jungmann 1959, 7) to be essential. Jungmann further recommends the work of Johannes Quasten, particularly Monumenta eucharistica et liturgica vetustissima in the Florilegium Patriscticum series (Bonn, 1937) (Jungmann 1959, 7).
Jungmann concludes that the study of historical liturgy allows us to rightly understand the past so as to carefully guide the future.