"This sacrament confers grace spiritually together with the virtue of charity… as Gregory observes in a Homily for Pentecost, 'God's love is never idle; for, wherever it is it does great works.' And consequently through this sacrament, as far as its power is concerned, not only is the habit of grace and of virtue bestowed, [...]
"This sacrament has of itself the power of bestowing grace… when the sacrament itself is really received, grace is increased, and the spiritual life perfected: yet in different fashion from the sacrament of Confirmation, in which grace is increased and perfected for resisting the outward assaults of Christ's enemies. But by this sacrament grace receives [...]
"The effect of this sacrament ought to be considered, first of all and principally, from what is contained in this sacrament, which is Christ; Who, just as by coming into the world, He visibly bestowed the life of grace upon the world, according to Jn. 1:17: 'Grace and truth came by Jesus Christ,' so also, [...]
"The Evangelists did not intend to hand down the forms of the sacraments, which in the primitive Church had to be kept concealed, as Dionysius observes at the close of his book on the ecclesiastical hierarchy; their object was to write the story of Christ. Nevertheless nearly all these words can be culled from various [...]
"The blood of Christ's Passion has its efficacy not merely in the elect among the Jews, to whom the blood of the Old Testament was exhibited, but also in the Gentiles; nor only in priests who consecrate this sacrament, and in those others who partake of it; but likewise in those for whom it is [...]
"It can be taken by way of metaphor, so that Christ's Passion is understood by the chalice by way of comparison, because, like a cup, it inebriates, according to Lam. 3:15: 'He hath filled me with bitterness, he hath inebriated me with wormwood': hence our Lord Himself spoke of His Passion as a chalice, when [...]
"In this sacrament the consecration of the matter consists in the miraculous change of the substance, which can only be done by God; hence the minister in performing this sacrament has no other act save the pronouncing of the words." Source: St. Thomas Aquinas, Summa Theologica
"The accidents continue in this sacrament without a subject. This can be done by Divine power: for since an effect depends more upon the first cause than on the second, God Who is the first cause both of substance and accident, can by His unlimited power preserve an accident in existence when the substance is [...]
"After what we have said above, it must be held most certainly that the whole Christ is under each sacramental species yet not alike in each. For the body of Christ is indeed present under the species of bread by the power of the sacrament, while the blood is there from real concomitance, as stated [...]
"His soul was truly separated from His body, as stated above. And therefore had this sacrament been celebrated during those three days when He was dead, the soul of Christ would not have been there, neither by the power of the sacrament, nor from real concomitance. But since 'Christ rising from the dead dieth now [...]