Catholics typically suppose that for the bread to be[come] the true body of Christ, a metaphysical change is required, that is, the "substance" of bread must be necessarily changed into the "substance" of Christ.
The point here is that Catholics almost always suppose that this (change of substance) is the ONLY way possible for the bread to be[come] the true body of Christ.
But we know that premise is not true. Indeed, some of Rome's own theologicans recognized this was not correct (but of course Rome tried to silence this fact with Mysterium Fidei).
Why do Catholics continue to entirely base their claims upon this false premise?
There is no false premise. Here is what some of the apostolic fathers have to say on the matter:
St. Ignatius of Antioch (c. 110 A.D.)
I have no taste for corruptible food nor for the pleasures of this life. I desire the Bread of God,
WHICH IS THE FLESH OF JESUS CHRIST, who was of the seed of David; and for drink
I DESIRE HIS BLOOD, which is love incorruptible.
(Letter to the Romans 7:3)
Take care, then, to use one Eucharist, so that whatever you do, you do according to God:
FOR THERE IS ONE FLESH OF OUR LORD JESUS CHRIST, and one cup
IN THE UNION OF HIS BLOOD; one
ALTAR, as there is one bishop with the presbytery…
(Letter to the Philadelphians 4:1)
They [i.e. the Gnostics] abstain from the Eucharist and from prayer, because they do not confess that
THE EUCHARIST IS THE FLESH OF OUR SAVIOR JESUS CHRIST, flesh which suffered for our sins and which the Father, in his goodness, raised up again.
(Letter to Smyrnians 7:1)
St. Justin the Martyr (c. 100 - 165 A.D.)
We call this food Eucharist; and no one else is permitted to partake of it, except one who believes our teaching to be true and who has been washed in the washing which is for the remission of sins and for regeneration [Baptism], and is thereby living as Christ has enjoined.
For not as common bread nor common drink do we receive these; but since Jesus Christ our Savior was made incarnate by the word of God and had both flesh and blood for our salvation, so too, as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by Him,
AND BY THE CHANGE OF WHICH our blood and flesh is nourished,
IS BOTH THE FLESH AND THE BLOOD OF THAT INCARNATED JESUS.
(First Apology, 66)
Moreover, as I said before, concerning the sacrifices which you at that time offered, God speaks through Malachi [1:10-12]…It is of the
SACRIFICES OFFERED TO HIM IN EVERY PLACE BY US, the Gentiles, that is,
OF THE BREAD OF THE EUCHARIST AND LIKEWISE OF THE CUP OF THE EUCHARIST, that He speaks at that time; and He says that we glorify His name, while you profane it.
(Dialogue with Trypho, 41)
St. Irenaeus of Lyons (c. 140 - 202 A.D.)
…He took from among creation that which is bread, and gave thanks, saying, "
THIS IS MY BODY." The cup likewise, which is from among the creation to which we belong,
HE CONFESSED TO BE HIS BLOOD.
He taught
THE NEW SACRIFICE OF THE NEW COVENANT, of which Malachi, one of the twelve prophets, had signified beforehand: [quotes Mal 1:10-11]. By these words He makes it plain that the former people will cease to make offerings to God;
BUT THAT IN EVERY PLACE SACRIFICE WILL BE OFFERED TO HIM, and indeed, a pure one; for His name is glorified among the Gentiles.
(Against Heresies 4:17:5)
But what consistency is there in those who hold that the bread over which thanks have been given
IS THE BODY OF THEIR LORD, and the cup
HIS BLOOD, if they do not acknowledge that He is the Son of the Creator… How can they say that the flesh which has been nourished
BY THE BODY OF THE LORD AND BY HIS BLOOD gives way to corruption and does not partake of life? …For as the bread from the earth, receiving the invocation of God,
IS NO LONGER COMMON BREAD BUT THE EUCHARIST, consisting of two elements, earthly and heavenly…
(Against Heresies 4:18:4-5)
If the
BODY be not saved, then, in fact, neither did the Lord redeem us with His
BLOOD; and neither is the cup of the
EUCHARIST THE PARTAKING OF HIS BLOOD nor is the bread which we break
THE PARTAKING OF HIS BODY…He has declared the cup, a part of creation,
TO BE HIS OWN BLOOD, from which He causes our blood to flow; and the bread, a part of creation,
HE HAS ESTABLISHED AS HIS OWN BODY, from which He gives increase to our bodies.
When, therefore, the mixed cup and the baked bread receives the Word of God and
BECOMES THE EUCHARIST, THE BODY OF CHRIST, and from these the substance of our flesh is increased and supported, how can they say that the flesh is not capable of receiving the gift of God,
WHICH IS ETERNAL LIFE -- flesh which is nourished
BY THE BODY AND BLOOD OF THE LORD…receiving the Word of God,
BECOMES THE EUCHARIST, WHICH IS THE BODY AND BLOOD OF CHRIST…
(Against Heresies 5:2:2-3)