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.1
Q 163. What are the parts of a sacrament?
A. The parts of a 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.2