First, we must consider what a sacrament is. It seems to me that a simple and proper definition would be to say that it is an outward sign by which the Lord seals on our consciences the promises of his good will toward us in order to sustain the weakness of our faith; and we in turn attest our piety toward him in the presence of the Lord and of his angels and before men. Here is another briefer definition: one may call it a testimony of divine grace toward us, confirmed by an outward sign, with mutual attestation of our piety toward him. Whichever of these definitions you may choose, it does not differ in meaning from that of Augustine, who teaches that a sacrament is “a visible sign of a sacred thing,” or “a visible form of an invisible grace,” but it better and more clearly explains the thing itself. For since there is something obscure in his brevity, in which many of the less educated are deceived, I have decided to give a fuller statement, using more words to dispel all doubt.
John Calvin | Institutes of the Christian Religion, ed. John T. McNeill, trans. Ford Lewis Battles. The Library of Christian Classics (Louisville: Westminster John Knox Press, 2011), 4.14.1.