Objection 1. It seems that the Eucharist is not a sacrament. For two sacraments ought not to be ordained for the same end, because every sacrament is efficacious in producing its effect. Therefore, since both Confirmation and the Eucharist are ordained for perfection, as Dionysius says (Eccl. Hier. iv), it seems that the Eucharist is not a sacrament, since Confirmation is one, as stated above (65, 1; 72, 1).
Objection 2. Further, in every sacrament of the New Law, that which comes visibly under our senses causes the invisible effect of the sacrament, just as cleansing with water causes the baptismal character and spiritual cleansing, as stated above (63, 6; 66, 1,3,7). But the species of bread and wine, which are the objects of our senses in this sacrament, neither produce Christ's true body, which is both reality and sacrament, nor His mystical body, which is the reality only in the Eucharist. Therefore, it seems that the Eucharist is not a sacrament of the New Law.
Objection 3. Further, sacraments of the New Law, as having matter, are perfected by the use of the matter, as Baptism is by ablution, and Confirmation by signing with chrism. If, then, the Eucharist be a sacrament, it would be perfected by the use of the matter, and not by its consecration. But this is manifestly false, because the words spoken in the consecration of the matter are the form of this sacrament, as will be shown later on (78, 1). Therefore the Eucharist is not a sacrament.
On the contrary, It is said in the Collect [Postcommunion "pro vivis et defunctis"]: "May this Thy Sacrament not make us deserving of punishment."
I answer that, The Church's sacraments are ordained for helping man in the spiritual life. But the spiritual life is analogous to the corporeal, since corporeal things bear a resemblance to spiritual. Now it is clear that just as generation is required for corporeal life, since thereby man receives life; and growth, whereby man is brought to maturity: so likewise food is required for the preservation of life. Consequently, just as for the spiritual life there had to be Baptism, which is spiritual generation; and Confirmation, which is spiritual growth: so there needed to be the sacrament of the Eucharist, which is spiritual food.
Reply to Objection 1. Perfection is twofold. The first lies within man himself; and he attains it by growth: such perfection belongs to Confirmation. The other is the perfection which comes to man from the addition of food, or clothing, or something of the kind; and such is the perfection befitting the Eucharist, which is the spiritual refreshment.
Reply to Objection 2. The water of Baptism does not cause any spiritual effect by reason of the water, but by reason of the power of the Holy Ghost, which power is in the water. Hence on John 5:4, "An angel of the Lord at certain times," etc., Chrysostom observes: "The water does not act simply as such upon the baptized, but when it receives the grace of the Holy Ghost, then it looses all sins." But the true body of Christ. bears the same relation to the species of the bread and wine, as the power of the Holy Ghost does to the water of Baptism: hence the species of the bread and wine produce no effect except from the virtue of Christ's true body.
Reply to Objection 3. A sacrament is so termed because it contains something sacred. Now a thing can be styled sacred from two causes; either absolutely, or in relation to something else. The difference between the Eucharist and other sacraments having sensible matter is that whereas the Eucharist contains something which is sacred absolutely, namely, Christ's own body; the baptismal water contains something which is sacred in relation to something else, namely, the sanctifying power: and the same holds good of chrism and such like. Consequently, the sacrament of the Eucharist is completed in the very consecration of the matter, whereas the other sacraments are completed in the application of the matter for the sanctifying of the individual. And from this follows another difference. For, in the sacrament of the Eucharist, what is both reality and sacrament is in the matter itself. but what is reality only, namely, the grace bestowed, is in the recipient; whereas in Baptism both are in the recipient, namely, the character, which is both reality and sacrament, and the grace of pardon of sins, which is reality only. And the same holds good of the other sacraments.
Source: Summa Theologica (New Advent)