This is from http://www.newadvent.org:
"The Church's Magna Charta, however, are the words of Institution, "This is my body — this is my blood", whose literal meaning she has uninterruptedly adhered to from the earliest times. The Real Presence is evinced, positively, by showing the necessity of the literal sense of these words, and negatively, by refuting the figurative interpretations. As regards the first, the very existence of four distinct narratives of the Last Supper, divided usually into the Petrine (Matthew 26:26 sqq.; Mark 14:22 sqq.) and the double Pauline accounts (Luke 22:19 sq.; 1 Corinthians 11:24 sq.), favors the literal interpretation. In spite of their striking unanimity as regards essentials, the Petrine account is simpler and clearer, whereas Pauline is richer in additional details and more involved in its citation of the words that refer to the Chalice. It is but natural and justifiable to expect that, when four different narrators in different countries and at different times relate the words of Institution to different circles of readers, the occurrence of an unusual figure of speech, as, for instance, that bread is a sign of Christ's Body, would, somewhere or other, betray itself, either in the difference of word-setting, or in the unequivocal expression of the meaning really intended, or at least in the addition of some such mark as: "He spoke, however, of the sign of His Body." But nowhere do we discover the slightest ground for a figurative interpretation. If, then, natural, literal interpretation were false, the Scriptural record alone would have to be considered as the cause of a pernicious error in faith and of the grievous crime of rendering Divine homage to bread (artolatria) — a supposition little in harmony with the character of the four Sacred Writers or with the inspiration of the Sacred Text. Moreover, we must not omit the important circumstance, that one of the four narrators has interpreted his own account literally. This is St. Paul (1 Corinthians 11:27 sq.), who, in the most vigorous language, brands the unworthy recipient as "guilty of body and of the blood of the Lord". There can be no question of a grievous offense against Christ Himself unless we suppose that the true Body and the true Blood of Christ are really present in the Eucharist. Further, if we attend only to the words themselves their natural sense is so forceful and clear that Luther wrote to the Christians of Strasburg in 1524: "I am caught, I cannot escape, the text is too forcible" (De Wette, II, 577). The necessity of the natural sense is not based upon the absurd assumption that Christ could not in general have resorted to use of figures, but upon the evident requirement of the case, which demand that He did not, in a matter of such paramount importance, have recourse to meaningless and deceptive metaphors. For figures enhance the clearness of speech only when the figurative meaning is obvious, either from the nature of the case (e.g. from a reference to a statue of Lincoln, by saying: "This is Lincoln") or from the usages of common parlance (e.g. in the case of this synecdoche: "This glass is wine"), Now, neither from the nature of the case nor in common parlance is bread an apt or possible symbol of the human body. Were one to say of a piece of bread: "This is Napoleon", he would not be using a figure, but uttering nonsense. There is but one means of rendering a symbol improperly so called clear and intelligible, namely, by, conventionally settling beforehand what it is to signify, as, for instance, if one were to say: "Let us imagine these two pieces of bread before us to be Socrates and Plato". Christ, however, instead of informing His Apostles that he intended to use such a figure, told them rather the contrary in the discourse containing the promise: "the bread that I will give, is my flesh, for the life of the world" (John 6:52), Such language, of course, could be used only by a God-man; so that belief in the Real Presence necessarily presupposes belief in the true Divinity of Christ, The foregoing rules would of themselves establish the natural meaning with certainty, even if the words of Institution, "This is my body — this is my blood", stood alone, But in the original text corpus (body) and sanguis (blood) are followed by significant appositional additions, the Body being designated as "given for you" and the Blood as "shed for you [many]"; hence the Body given to the Apostles was the self same Body that was crucified on Good Friday, and the Chalice drunk by them, the self same Blood that was shed on the Cross for our sins, Therefore the above-mentioned appositional phrases directly exclude every possibility of a figurative interpretation."
Again, I am not here to evangelize.
