What is a sacrament and what is its function?
A general answer, presented by McGrath, is that a sacrament is an external sign or rite that brings believers together and strengthens faith (McGrath:400). More particular definitions have been made by the church throughout the ages. What follows are some of the most important ones.
Third century: Tertullian paralleled sacraments to military oaths making them important commitments of loyalty to the Christian church at a time of great persecution.
Fourth century: Augustine of Hippo defined sacraments as “visible forms of invisible grace”, they both signify grace, by likeness, and enable it for us, by divine connection, like a bridge over the gap between us and God.
Twelfth century: Hugh of St. Victor (1096-1141) made Augustine’s definition more precise by making sacraments an element perceivable by our senses that represents grace by likeness, signifies it by institution and contains it by sanctification. The sacraments of the New Covenant with Christ actually confers grace, unlike the Old Covenant whose sacraments were signs of that grace.
Peter Lombard developed Hugh’s characterization further into a definition that then remained unchallenged until the Reformation and still stands in the Catholic Church: Sacraments bear the image, or likeness, of the grace that it signifies and causes. They signify and sanctify, if both of these functions are not present in an act, it is not a sacrament. (McGrath:401-404)
Sixteenth century/Reformation: Luther defined sacrament as the Word added to an external sign. Sacraments are only those symbolic acts that are instituted by God with His promise: baptism and the Lord’s Supper. Their purpose is faith. The sacrament is a manifestation of the Word before our eyes, so that we may believe more completely, and it is also directed to us personally, so that we may believe that God’s promise is for each and everyone of us. As an external act done to our bodies, it ensures in our faith our complete salvation, body and soul. It does not, however, perform salvation by its execution itself, for that is done by Jesus Christ. As we receive the Word in faith, similarly we must receive the sacraments in faith. The Word and faith is superior to the sacraments so that one who has faith and hears the Word actually does not need to be externally baptised. This is because the Word contains everything, it lacks nothing, the sacrament on the other hand, is not self-sufficient, it depends on the Word that instituted it and the faith that receives it. Because the Word is the primary factor, the sacrament is nothing without it, but as far as faith is concerned, the sacrament can come before faith, because it calls man to faith and can establish it in his heart. The performer of the sacrament is God, not the priest nor our self, so it works by His power even though we and the priest may not believe, but then the promise behind it remains unreceived. It is faith that receives the sacrament. (Althaus:345-352)
The general function of the sacraments can be discerned from the above definitions. Sacraments convey grace (1); strengthen faith (2); enhance the commitment and unity of the church (3); and reassure us of God’s promises toward us (4). Another four-fold way to put these functions are that the sacraments (here especially the Eucharist, the Lord’s Supper) are a recollection of the past (1) and an anticipation of the future (2), they affirm our individual faith (3) and uphold corporate belonging (4). How all these functions are brought forth and what their ranks are, is considered differently by different theologians, the main controversy being between Protestants and Catholics, the former refuting the latter’s ex opere operato, that the sacramental acts are in themselves potent, regardless of the belief or knowledge of the receiver/performer.Source: Althaus, Paul, The Theology of Martin Luther, Philadelphia: Fortress Press 1966. McGrath, Alister E. Christian Theology – An Introduction. Oxford: Blackwell 2011.