Objection 1. It seems that a sacrament is a sign of one thing only. For that which signifies many things is an ambiguous sign, and consequently occasions deception: this is clearly seen in equivocal words. But all deception should be removed from the Christian religion, according to Colossians 2:8: "Beware lest any man cheat you by philosophy and vain deceit." Therefore it seems that a sacrament is not a sign of several things.
Objection 2. Further, as stated above (Article 2), a sacrament signifies a holy thing in so far as it makes man holy. But there is only one cause of man's holiness, viz. the blood of Christ; according to Hebrews 13:12: "Jesus, that He might sanctify the people by His own blood, suffered without the gate." Therefore it seems that a sacrament does not signify several things.
Objection 3. Further, it has been said above (2, ad 3) that a sacrament signifies properly the very end of sanctification. Now the end of sanctification is eternal life, according to Romans 6:22: "You have your fruit unto sanctification, and the end life everlasting." Therefore it seems that the sacraments signify one thing only, viz. eternal life.
On the contrary, In the Sacrament of the Altar, two things are signified, viz. Christ's true body, and Christ's mystical body; as Augustine says (Liber Sent. Prosper.).
I answer that, As stated above (Article 2) a sacrament properly speaking is that which is ordained to signify our sanctification. In which three things may be considered; viz. the very cause of our sanctification, which is Christ's passion; the form of our sanctification, which is grace and the virtues; and the ultimate end of our sanctification, which is eternal life. And all these are signified by the sacraments. Consequently a sacrament is a sign that is both a reminder of the past, i.e. the passion of Christ; and an indication of that which is effected in us by Christ's passion, i.e. grace; and a prognostic, that is, a foretelling of future glory.
Reply to Objection 1. Then is a sign ambiguous and the occasion of deception, when it signifies many things not ordained to one another. But when it signifies many things inasmuch as, through being mutually ordained, they form one thing, then the sign is not ambiguous but certain: thus this word "man" signifies the soul and body inasmuch as together they form the human nature. In this way a sacrament signifies the three things aforesaid, inasmuch as by being in a certain order they are one thing.
Reply to Objection 2. Since a sacrament signifies that which sanctifies, it must needs signify the effect, which is implied in the sanctifying cause as such.
Reply to Objection 3. It is enough for a sacrament that it signify that perfection which consists in the form, nor is it necessary that it should signify only that perfection which is the end.
Source: Summa Theologica (New Advent)