Senn, Frank C. "Chapter Two: The Incarnational Reality of Christian Liturgy." Christian Liturgy: Catholic and Evangelical. Minneapolis: Fortress Press, 1997, 30-49.
Senn observes that religious rites contain symbols which connect the rite with deeply human ways of knowing and experiencing ideas (Senn 1997, 30). Christianity uses a great number of these ritual symbols. Augustine referred to the sacrament as a visible word created by God's word coming to a visible element, and would use the word "sacrament" of many actions (Senn 1997, 31).
Theological distinctions between sacrament and sacrifice have long been recognized. Senn is clear that here, "sacrifice" does not mean that which atones for sin but rather that which the Christian does in thanksgiving to God. The "sacrament," on the contrary, is what God provides for the good of His people (Senn 1997, 32). Senn does note that many actions in Christian practice have elements both of sacrament and sacrifice. They cannot be separated entirely (Senn 1997, 34). Chrsit routinely acts by means of words in our mortal mouth and actions performed by our hands.
In Christian piety, Christ plays a central role in sacramental encounters (Senn 1997, 36). Christ as the mediator between God and man not only receives human prayers and other sacrificial works on behalf of God, but delivers the sacramental blessings from God to humans. Senn traces this to very early in Christianity. He further considers that the language was clarified in the fact of heresy, not because the orthodox understanding changed, but because a clearer articulation was needed (Senn 1997, 37).
Because ritual and liturgy include both the actions of God and the actions of humans, Senn proposes that a study of liturgy must include theology and various human social sciences such as "anthropology, comparative religion, psychology, and sociology" (Senn 1997, 40). An understanding of both God and man is essential. There is an additional element dealing with the historical and theological use of symbolic elements (Senn 1997, 41), though I personally would classify this as history or anthropology. Senn's point, however, is helpful. Elements of liturgical ritual have a tendency to change slowly or even stop changing even when their original significance is no longer valid. Liturgy is static in nature, rather than being innovative. Senn finds that in a way liturgy is canonical in nature. It remains as presented (Senn 1997, 43).
Senn reminds the reader that liturgy serves the Gospel. In this it participates in the freedom which Christians have ascribed to the Gospel, as well as providing necessary structure in the life of the Church (Senn 1997, 43-44). Christian worship has been remarkably constant over the course of approximately 2,000 years. The constancy Senn considers to be catholicity. It is applied to the whole church over time (Senn 1997, 45). At the same time, the essential elements of the liturgy proclaim the unchanging message of the Gospel - Christ overcoming death for sinners, gathering them as the people of God. This is the evangelical, or gospel-centered, aspect of the liturgy.