I found some biblical text: why does Protestantism contradict its plain teaching?
I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world.
Truly, truly, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you.
Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.
For my flesh is meat indeed, and my blood is drink indeed. He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him.
Christ the Son of God instituted the sacrament of the Eucharist - the Bread and Wine becoming His Body and Blood - at the Last Supper.
And he took bread, and when he had given thanks, he broke it and gave it to them, saying, “This is my body, which is given for you. Do this in remembrance of me.” And likewise the cup after they had eaten, saying, “This cup that is poured out for you is the new covenant in my blood."
If any corroborating evidence were needed, St Paul speaks about the Eucharist in Corinthians.
And when he had given thanks, he broke it, and said, “This is my body which is for you. Do this in remembrance of me.” In the same way also he took the cup, after supper, saying, “This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.”
For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes. Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty concerning the body and blood of the Lord.
Why does Protestantism contradict the plain teaching of these Biblical texts?
“It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.” (John 6:63)
With this one verse, Jesus Christ completely denied the “transubstantial” sense of His Words. The flesh profits nothing. He as much tells us that His Words were meant to be taken figuratively - and that sets the parameters for all further intepretation on this subject appearing later in the epistles.
Indeed, we should observe that the disciples who turned back and left Him were the ones who thought He was literally telling them to eat His flesh and drink His blood. The ones who stayed faithful were the ones who understood that He was speaking spiritually and figuratively.
And this isn’t even getting into the theme that appears across Scripture of “eating the Word of God” that is always elsewhere used in a symbolic or figurative sense.