The Catholic teaching of transubstantiation is based chiefly on the words of Jesus to his disciples, when He instituted the Sacrament of the Lord’s Supper:
26 And as they were eating, Jesus took bread, and blessed it, and brake it, and gave it to the disciples, and said, Take, eat; this is my body.
27 And he took the cup, and gave thanks, and gave it to them, saying, Drink ye all of it;
28 For this is my blood of the new testament, which is shed for many for the remission of sins.
22 And as they did eat, Jesus took bread, and blessed, and brake it, and gave to them, and said, Take, eat: this is my body.
23 And he took the cup, and when he had given thanks, he gave it to them: and they all drank of it.
24 And he said unto them, This is my blood of the new testament, which is shed for many.
But these are extremely weak and flimsy arguments. In these verses Jesus is not saying that the bread and wine are literally his flesh and blood. He is using hyperbole (a form of rhetorical exaggeration) which is extremely common in Semitic cultures and languages. Even today if you talk to an Arab in his native language, you will find that he makes much use of hyperbole in his everyday speech. The Bible too is full of this kind of rhetorical device. Here are some examples:
Matthew 5:29 And if thy right eye offend thee, pluck it out, and cast it from thee…
This does not mean to literally pluck out your own eye; it means to be ruthless at suppressing and eliminating the cause of temptation.
Matthew 6:3 But when thou doest alms, let not thy left hand know what thy right hand doeth.
This does not mean that one should literally not let his left hand know what his other hand is doing. It means to give alms discreetly, so that you will not be doing it for a show or pretence, to boast or flout your “charitableness”.
Matthew 11:23 And thou,
This does not mean that
Matthew 23:24 Ye blind guides, which strain at a gnat, and swallow a camel.
This does not mean that they were literally “swallowing a camel”. It means paying too much attention to unimportant detail, while ignoring the more important considerations.
Luke 14:26 If any man come to me, and hate not his father, and mother, and wife, and children, and brethren, and sisters, yea, and his own life also, he cannot be my disciple.
This does not mean that one should literally hate one’s father and mother, and brothers and sisters, and wife and kids in order to become a disciple of Jesus. It means that one should be willing to put God above all other considerations.
John 12:19 The Pharisees therefore said among themselves … behold, the world is gone after him.
This did not mean that literally the whole world had gone after Jesus; but it means that a large number of Israelites had done so, enough to have the Pharisees worried.
John 17:11 And now I am no more in the world, but these are in the world, and I come to thee.
This did not mean that Jesus was literally “no more in the world;” but that He was anticipating His death, which was shortly to take place.
A false reading of these passages can have serious consequences sometimes. The great third century theologian Origen is known to have emasculated himself in his youth out of a literal reading of Matthew 19:12; a costly mistake which he is said to have regretted later in his life.
In the verses quoted earlier, they do not mean that the bread and wine were literally His flesh and blood; or that one should literally eat His flesh and blood in order to be saved. The bread and wine are emblems of His flesh and blood. They are a memorial of His sufferings, to be taken in remembrance of his sacrifice. Nothing more or less is to be understood by them. See scriptures cited in my previous post: Do Catholics Sacrifice Jesus Christ At Mass?
Interestingly, in the following verses Jesus Himself actually explains the meaning of His own hyperbole, so that no one need have any excuse for misunderstanding them:
53 Then Jesus said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you.
54 Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.
55 For my flesh is meat indeed, and my blood is drink indeed.
56 He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him.
57 As the living Father hath sent me, and I live by the Father: so he that eateth me, even he shall live by me.
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60 Many therefore of his disciples, when they had heard this, said, This is an hard saying; who can hear it?
61 When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you?
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63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
In other words, those words were not meant to be taken literally, but spiritually and metaphorically, for it is the words that he speaks that are “spirit and life,” not the literal eating of His flesh and blood. He couldn’t have made it clearer than that.