And Christ says that the opposite are anathema. I'll stick with Christ -- you can leave as some did in the Bible.
Gospel Jn 6:51-58
Jesus said to the Jewish crowds:
"I am the living bread that came down from heaven;
whoever eats this bread will live forever;
and the bread that I will give
is my flesh for the life of the world."
The Jews quarreled among themselves, saying,
"How can this man give us his flesh to eat?"
Jesus said to them,
"Amen, amen, I say to you,
unless you eat the flesh of the Son of Man and drink his blood,
you do not have life within you.
Whoever eats my flesh and drinks my blood
has eternal life,
and I will raise him on the last day.
For my flesh is true food,
and my blood is true drink.
Whoever eats my flesh and drinks my blood
remains in me and I in him.
Just as the living Father sent me
and I have life because of the Father,
so also the one who feeds on me
will have life because of me.
This is the bread that came down from heaven.
Unlike your ancestors who ate and still died,
whoever eats this bread will live forever."
Trent is stating that those who deny transubstantiation are the ones who are anathema. I often get confused when I read them too.
Can an unbeliever take the Eucharist from a non-catholic preacher and gain eternal life?
Clement of Alexandria wrote the following about John 6 :
Elsewhere the Lord, in the Gospel according to John, brought this out by symbols, when He said: “Eat ye my flesh, and drink my blood,” describing distinctly by metaphor the drinkable properties of faith and the promise, by means of which the Church, like a human being consisting of many members, is refreshed and grows, is welded together and compacted of both,—of faith, which is the body, and of hope, which is the soul; as also the Lord of flesh and blood. For in reality the blood of faith is hope, in which faith is held as by a vital principle. (The Instructor, 1:6)
Augustine
Vol. II, On Christian Doctrine, Book III, Chapter 16 (section 24).
If the sentence is one of command, either forbidding a crime or vice, or enjoining an act of prudence or benevolence, it is not figurative. If, however, it seems to enjoin a crime or vice, or to forbid an act of prudence or benevolence, it is figurative. “Except ye eat the flesh of the Son of man,” says Christ, “and drink His blood, ye have no life in you.” This seems to enjoin a crime or a vice; it is therefore a figure, enjoining that we should have a share in the sufferings of our Lord, and that we should retain a sweet and profitable memory of the fact that His flesh was wounded and crucified for us.