They got this from Aristotle who taught that all matter consists of "accidents" and "substance". Accidents are the outward appearance of an object and substance is its inner essence.
The theory of transubstantiation says that at the consecration of the Mass, the substance of the bread and wine change while their accidents remain the same.
1374 The mode of Christ's presence under the Eucharistic species is unique. It raises the Eucharist above all the sacraments as "
the perfection of the spiritual life and the end to which all the sacraments tend."201 In the most blessed sacrament of the Eucharist "the body and blood, together with the soul and divinity, of our Lord Jesus Christ and, therefore,
the whole Christ is truly, really, and substantially contained."202 "This presence is called 'real' - by which is not intended to exclude the other types of presence as if they could not be 'real' too, but because it is presence in the fullest sense: that is to say, it is a
substantial presence by which Christ, God and man, makes himself wholly and entirely present."203
1375 It is by the conversion of the bread and wine into Christ's body and blood that Christ becomes present in this sacrament. The Church Fathers strongly affirmed the faith of the Church in the efficacy of the Word of Christ and of the action of the Holy Spirit to bring about this conversion.
Thus St. John Chrysostom declares:
It is not man that causes the things offered to become the Body and Blood of Christ, but he who was crucified for us, Christ himself. The priest, in the role of Christ, pronounces these words, but their power and grace are God's. This is my body, he says. This word transforms the things offered.204
And St. Ambrose says about this conversion:
Be convinced that this is not what nature has formed, but what the blessing has consecrated. The power of the blessing prevails over that of nature, because by the blessing nature itself is changed. . . . Could not Christ's word, which can make from nothing what did not exist, change existing things into what they were not before?
It is no less a feat to give things their original nature than to change their nature.205
1376 The Council of Trent summarizes the Catholic faith by declaring: "Because Christ our Redeemer said that it was truly his body that he was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change of the whole
substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood. This change the holy Catholic Church has fittingly and properly called transubstantiation."206
1377 The Eucharistic presence of Christ begins at the moment of the consecration and endures as long as the Eucharistic species subsist.
Christ is present whole and entire in each of the species and whole and entire in each of their parts, in such a way that the breaking of the bread does not divide Christ.207
The second problem with this interpretation is that it requires the eating of human flesh. For a Jew to drink human blood would have been more than repulsive (according to the Mosaic Law, it would have been unlawful). Where was the heated discussion about drinking blood and eating flesh? But, this is what the disciples did at the Last Supper. They ate human flesh and drank human blood.
Paul proclaimed later, in Acts 10:14
"But Peter said, Not so, Lord; for
I have never eaten any thing that is common or unclean."
Acts_17:22 Then Paul stood in the midst of Mars' hill, and said, Ye men of Athens,
I perceive that in all things ye are too superstitious.
