Objection 1: It seems that not every sign of a sacred thing is a sacrament. For all sensible creatures are signs of sacred things; according to Rom. 1:20: "The invisible things of God are clearly seen being understood by the things that are made." And yet all sensible things cannot be called sacraments. Therefore not every sign of a sacred thing is a sacrament.
Obj. 2: Further, whatever was done under the Old Law was a figure of Christ Who is the "Holy of Holies" (Dan. 9:24), according to 1 Cor. 10:11: "All (these) things happened to them in figure"; and Col. 2:17: "Which are a shadow of things to come, but the body is Christ's." And yet not all that was done by the Fathers of the Old Testament, not even all the ceremonies of the Law, were sacraments, but only in certain special cases, as stated in the Second Part (I-II, Q. 101, A. 4). Therefore it seems that not every sign of a sacred thing is a sacrament.
Obj. 3: Further, even in the New Testament many things are done in sign of some sacred thing; yet they are not called sacraments; such as sprinkling with holy water, the consecration of an altar, and such like. Therefore not every sign of a sacred thing is a sacrament.
On the contrary, A definition is convertible with the thing defined. Now some define a sacrament as being "the sign of a sacred thing"; moreover, this is clear from the passage quoted above (A. 1) from Augustine. Therefore it seems that every sign of a sacred thing is a sacrament.
I answer that, Signs are given to men, to whom it is proper to discover the unknown by means of the known. Consequently a sacrament properly so called is that which is the sign of some sacred thing pertaining to man; so that properly speaking a sacrament, as considered by us now, is defined as being the "sign of a holy thing so far as it makes men holy."
Reply Obj. 1: Sensible creatures signify something holy, viz. Divine wisdom and goodness inasmuch as these are holy in themselves; but not inasmuch as we are made holy by them. Therefore they cannot be called sacraments as we understand sacraments now.
Reply Obj. 2: Some things pertaining to the Old Testament signified the holiness of Christ considered as holy in Himself. Others signified His holiness considered as the cause of our holiness; thus the sacrifice of the Paschal Lamb signified Christ's Sacrifice whereby we are made holy: and such like are properly styled sacraments of the Old Law.
Reply Obj. 3: Names are given to things considered in reference to their end and state of completeness. Now a disposition is not an end, whereas perfection is. Consequently things that signify disposition to holiness are not called sacraments, and with regard to these the objection is verified: only those are called sacraments which signify the perfection of holiness in man.