WLC Q.163: What are the parts of the sacrament?
The parts of the sacrament are two; the one an outward and sensible sign, used according to Christ’s own appointment; the other an inward and spiritual grace thereby signified. (Matt. 3:11, 1 Pet. 3:21, Rom. 2:28–29)
There is, in every sacrament, a spiritual relation, or sacramental union, between the sign and the thing signified: whence it comes to pass, that the names and effects of the one are attributed to the other. (Gen. 17:10, Matt. 26:27–28, Tit. 3:5)