"Because the change of the bread and wine is not terminated at the Godhead or the soul of Christ, it follows as a consequence that the Godhead or the soul of Christ is in this sacrament not by the power of the sacrament, but from real concomitance. For since the Godhead never set aside the [...]
"It is evident to sense that all the accidents of the bread and wine remain after the consecration. And this is reasonably done by Divine providence. First of all, because it is not customary, but horrible, for men to eat human flesh, and to drink blood. And therefore Christ's flesh and blood are set before [...]
"It is absolutely necessary to confess according to Catholic faith that the entire Christ is in this sacrament." Source: St. Thomas Aquinas, Summa Theologica
"And St. Thomas, in common with other philosophers, acknowledges that friendship is a virtue, and he certainly means individual friendships, because he says that we cannot bestow perfect friendship on many persons." Source: St. Francis de Sales, Introduction to the Devout Life, Random House, Inc., 2002.
"Augustine says in his book on the Sentences of Prosper (Lanfranc, De Corp. et Sang. Dom. xiii): 'Under the species which we behold, of bread and wine, we honor invisible things, i.e. flesh and blood.'" Source: St. Thomas Aquinas, Summa Theologica
"Form cannot be changed into form, nor matter into matter by the power of any finite agent. Such a change, however, can be made by the power of an infinite agent, which has control over all being, because the nature of being is common to both forms and to both matters; and whatever there is [...]
"Consequently, no natural or created agent can act except by changing the form in something; and on this account every change made according to nature's laws is a formal change. But God is infinite act… hence His action extends to the whole nature of being. Therefore He can work not only formal conversion, so that [...]
"Some have held that the substance of the bread and wine remains in this sacrament after the consecration. But this opinion cannot stand: first of all, because by such an opinion the truth of this sacrament is destroyed, to which it belongs that Christ's true body exists in this sacrament; which indeed was not there [...]
"Some men accordingly, not paying heed to these things, have contended that Christ's body and blood are not in this sacrament except as in a sign, a thing to be rejected as heretical, since it is contrary to Christ's words." Source: St. Thomas Aquinas, Summa Theologica
"The presence of Christ's true body and blood in this sacrament cannot be detected by sense, nor understanding, but by faith alone, which rests upon Divine authority. Hence, on Lk. 22:19: 'This is My body which shall be delivered up for you,' Cyril says: 'Doubt not whether this be true; but take rather the Saviour's [...]