The Doctrine of the Holy Eucharist simply states that Jesus is literally and wholly present—body and blood, soul and divinity—under the appearances of bread and wine. This was attested by all Church Fathers. Some have even died for it.
1 Cor. 10:16–17:
16 The cup of blessing that we bless, is it not a sharing in the blood of Christ? The bread that we break, is it not a sharing in the body of Christ? 17 Because there is one bread, we who are many are one body, for we all partake of the one bread.
1 Cor. 11:23–29:
The Institution of the Lord’s Supper
23 For I received from the Lord what I also handed on to you, that the Lord Jesus on the night when he was betrayed took a loaf of bread, 24 and when he had given thanks, he broke it and said, “This is my body that is for[g] you. Do this in remembrance of me.” 25 In the same way he took the cup also, after supper, saying, “This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.” 26 For as often as you eat this bread and drink the cup, you proclaim the Lord’s death until he comes.
Partaking of the Supper Unworthily
27 Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be answerable for the body and blood of the Lord.28 Examine yourselves, and only then eat of the bread and drink of the cup.29 For all who eat and drink[h] without discerning the body,[i] eat and drink judgment against themselves. 30 For this reason many of you are weak and ill, and some have died.[j] 31 But if we judged ourselves, we would not be judged.32 But when we are judged by the Lord, we are disciplined[k] so that we may not be condemned along with the world.
and John 6:32–69:
31 Our ancestors ate the manna in the wilderness; as it is written, ‘He gave them bread from heaven to eat.’” 32 Then Jesus said to them, “Very truly, I tell you, it was not Moses who gave you the bread from heaven, but it is my Father who gives you the true bread from heaven. 33 For the bread of God is that which comes down from heaven and gives life to the world.” 34 They said to him, “Sir, give us this bread always.”
35 Jesus said to them, “I am the bread of life. Whoever comes to me will never be hungry, and whoever believes in me will never be thirsty. 36 But I said to you that you have seen me and yet do not believe. 37 Everything that the Father gives me will come to me, and anyone who comes to me I will never drive away; 38 for I have come down from heaven, not to do my own will, but the will of him who sent me. 39 And this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up on the last day.40 This is indeed the will of my Father, that all who see the Son and believe in him may have eternal life; and I will raise them up on the last day.”
41 Then the Jews began to complain about him because he said, “I am the bread that came down from heaven.” 42 They were saying, “Is not this Jesus, the son of Joseph, whose father and mother we know? How can he now say, ‘I have come down from heaven’?” 43 Jesus answered them, “Do not complain among yourselves. 44 No one can come to me unless drawn by the Father who sent me; and I will raise that person up on the last day. 45 It is written in the prophets, ‘And they shall all be taught by God.’ Everyone who has heard and learned from the Father comes to me. 46 Not that anyone has seen the Father except the one who is from God; he has seen the Father. 47 Very truly, I tell you, whoever believes has eternal life. 48 I am the bread of life. 49 Your ancestors ate the manna in the wilderness, and they died. 50 This is the bread that comes down from heaven, so that one may eat of it and not die. 51 I am the living bread that came down from heaven. Whoever eats of this bread will live forever; and the bread that I will give for the life of the world is my flesh.”
52 The Jews then disputed among themselves, saying, “How can this man give us his flesh to eat?” 53 So Jesus said to them, “Very truly, I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you.54 Those who eat my flesh and drink my blood have eternal life, and I will raise them up on the last day; 55 for my flesh is true food and my blood is true drink.56 Those who eat my flesh and drink my blood abide in me, and I in them.57 Just as the living Father sent me, and I live because of the Father, so whoever eats me will live because of me. 58 This is the bread that came down from heaven, not like that which your ancestors ate, and they died. But the one who eats this bread will live forever.” 59 He said these things while he was teaching in the synagogue at Capernaum.
60 When many of his disciples heard it, they said, “This teaching is difficult; who can accept it?” 61 But Jesus, being aware that his disciples were complaining about it, said to them, “Does this offend you? 62 Then what if you were to see the Son of Man ascending to where he was before? 63 It is the spirit that gives life; the flesh is useless. The words that I have spoken to you are spirit and life.64 But among you there are some who do not believe.” For Jesus knew from the first who were the ones that did not believe, and who was the one that would betray him. 65 And he said, “For this reason I have told you that no one can come to me unless it is granted by the Father.”
66 Because of this many of his disciples turned back and no longer went about with him. 67 So Jesus asked the twelve, “Do you also wish to go away?”68 Simon Peter answered him, “Lord, to whom can we go? You have the words of eternal life. 69 We have come to believe and know that you are the Holy One of God.”