"The Paschal Lamb foreshadowed this sacrament in these three ways. First of all, because it was eaten with unleavened loaves, according to Ex. 12:8: 'They shall eat flesh . . . and unleavened bread.' As to the second because it was immolated by the entire multitude of the children of Israel on the fourteenth day [...]
"Because last words, chiefly such as are spoken by departing friends, are committed most deeply to memory; since then especially affection for friends is more enkindled, and the things which affect us most are impressed the deepest in the soul. Consequently, since, as Pope Alexander I says, 'among sacrifices there can be none greater [...]
"This sacrament has a threefold significance. One with regard to the past… it is commemorative of our Lord's Passion, which was a true sacrifice… and in this respect it is called a 'Sacrifice.' With regard to the present… that of Ecclesiastical unity, in which men are aggregated through this Sacrament; and in this respect it [...]
"This sacrament was appropriately instituted at the supper, when Christ conversed with His disciples for the last time. First of all, because of what is contained in the sacrament: for Christ is Himself contained in the Eucharist sacramentally. Consequently, when Christ was going to leave His disciples in His proper species, He left Himself with [...]
"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." Source: St. Thomas Aquinas, Summa Theologica [...]
St. Thomas discusses a spiritual Communion, where we are unable to receive the Eucharist, but through a true desire for union with Christ, we can receive spiritually. Ideally, we would go to Mass every day and receive the Blessed Sacrament every day, but for most, that is not possible. Instead, we can desire and seek [...]
"A thing is one in perfection, when it is complete through the presence of all that is needed for its end; as a man is complete by having all the members required for the operation of his soul, and a house by having all the parts needful for dwelling therein. And so this sacrament is [...]
"The Apostle says (1 Cor. 10:17): "For we, being many, are one bread, one body, all that partake of one bread": from which it is clear that the Eucharist is the sacrament of the Church's unity." Source: St. Thomas Aquinas, Summa Theologica
"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 [...]
"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." [...]