Objection 1. It seems that this sacrament is necessary for salvation. For our Lord said (John 6:54): "Except you eat the flesh of the Son of Man, and drink His blood, you shall not have life in you." But Christ's flesh is eaten and His blood drunk in this sacrament. Therefore, without this sacrament man cannot have the health of spiritual life.
Objection 2. Further, this sacrament is a kind of spiritual food. But bodily food is requisite for bodily health. Therefore, also is this sacrament, for spiritual health.
Objection 3. Further, as Baptism is the sacrament of our Lord's Passion, without which there is no salvation, so also is the Eucharist. For the Apostle says (1 Corinthians 11:26): "For as often as you shall eat this bread, and drink the chalice, you shall show the death of the Lord, until He come." Consequently, as Baptism is necessary for salvation, so also is this sacrament.
On the contrary, Augustine writes (Ad Bonifac. contra Pelag. I): "Nor are you to suppose that children cannot possess life, who are deprived of the body and blood of Christ."
I answer that, Two things have to be considered in this sacrament, namely, the sacrament itself, and what is contained in it. Now it was stated above (1, Objection 2) that the reality of the sacrament is the unity of the mystical body, without which there can be no salvation; for there is no entering into salvation outside the Church, just as in the time of the deluge there was none outside the Ark, which denotes the Church, according to 1 Peter 3:20-21. And it has been said above (Question 68, Article 2), that before receiving a sacrament, the reality of the sacrament can be had through the very desire of receiving the sacrament. Accordingly, before actual reception of this sacrament, a man can obtain salvation through the desire of receiving it, just as he can before Baptism through the desire of Baptism, as stated above (Question 68, Article 2). Yet there is a difference in two respects. First of all, because Baptism is the beginning of the spiritual life, and the door of the sacraments; whereas the Eucharist is, as it were, the consummation of the spiritual life, and the end of all the sacraments, as was observed above (Question 63, Article 6): for by the hallowings of all the sacraments preparation is made for receiving or consecrating the Eucharist. Consequently, the reception of Baptism is necessary for starting the spiritual life, while the receiving of the Eucharist is requisite for its consummation; by partaking not indeed actually, but in desire, as an end is possessed in desire and intention. Another difference is because by Baptism a man is ordained to the Eucharist, and therefore from the fact of children being baptized, they are destined by the Church to the Eucharist; and just as they believe through the Church's faith, so they desire the Eucharist through the Church's intention, and, as a result, receive its reality. But they are not disposed for Baptism by any previous sacrament, and consequently before receiving Baptism, in no way have they Baptism in desire; but adults alone have: consequently, they cannot have the reality of the sacrament without receiving the sacrament itself. Therefore this sacrament is not necessary for salvation in the same way as Baptism is.
Reply to Objection 1. As Augustine says, explaining John 6:54, "This food and this drink," namely, of His flesh and blood: "He would have us understand the fellowship of His body and members, which is the Church in His predestinated, and called, and justified, and glorified, His holy and believing ones." Hence, as he says in his Epistle to Boniface (Pseudo-Beda, in 1 Corinthians 10:17): "No one should entertain the slightest doubt, that then every one of the faithful becomes a partaker of the body and blood of Christ, when in Baptism he is made a member of Christ's body; nor is he deprived of his share in that body and chalice even though he depart from this world in the unity of Christ's body, before he eats that bread and drinks of that chalice."
Reply to Objection 2. The difference between corporeal and spiritual food lies in this, that the former is changed into the substance of the person nourished, and consequently it cannot avail for supporting life except it be partaken of; but spiritual food changes man into itself, according to that saying of Augustine (Confess. vii), that he heard the voice of Christ as it were saying to him: "Nor shalt thou change Me into thyself, as food of thy flesh, but thou shalt be changed into Me." But one can be changed into Christ, and be incorporated in Him by mental desire, even without receiving this sacrament. And consequently the comparison does not hold.
Reply to Objection 3. Baptism is the sacrament of Christ's death and Passion, according as a man is born anew in Christ in virtue of His Passion; but the Eucharist is the sacrament of Christ's Passion according as a man is made perfect in union with Christ Who suffered. Hence, as Baptism is called the sacrament of Faith, which is the foundation of the spiritual life, so the Eucharist is termed the sacrament of Charity, which is "the bond of perfection" (Colossians 3:14).
Source: Summa Theologica (New Advent)