The Eucharist was seen as the most important part of the christian liturgy, of the christian religion and of the christian cult since the beginning, it is vital and it is crucial. Christ knew it when He instituted it, all the christians, the saints, the Fathers saw it that way until the Protestant Reform when they ditched the most important and crucial part of christianity.
Matthey 26:26-28 -
26 And as they were eating, Jesus took bread, blessed and broke it, and gave it to the disciples and said, Take, eat; this is My body.
27 Then He took the cup, and gave thanks, and gave it to them, saying, Drink from it, all of you. 28 For this is My blood of the new covenant, which is shed for many for the remission of sins.
We find the same in Mark chapter 14:22-24.
Luke 22, 19-20
19 And He took bread, gave thanks and broke it, and gave it to them, saying, This is My body which is given for you; do this in remembrance of Me.
20 Likewise He also took the cup after supper, saying, This cup is the new covenant in My blood, which is shed for you.
John 6, 51-56 offers us the most clear statement that the bread and the body is TRULY the Body of our Lord:
51 I am the living bread which came down from heaven. If anyone eats of this bread, he will live forever; and the bread that I shall give is My flesh, which I shall give for the life of the world.
52 The Jews therefore quarreled among themselves, saying, How can this Man give us His flesh to eat?
53 Then Jesus said to them, Most assuredly, I say to you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in you. 54 Whoever eats My flesh and drinks My blood has eternal life, and I will raise him up at the last day. 55 For My flesh is food indeed, and My blood is drink indeed. 56 He who eats My flesh and drinks My blood abides in Me, and I in him.
But not only Jesus said it (if that wasn't enough), the Apostles considered it to be real too:
1 Corinthians 11:24-25
24 and when He had given thanks, He broke it and said, Take, eat; this is My body which is broken for you; do this in remembrance of Me. 25 In the same manner He also took the cup after supper, saying, This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me.
27:30
27 Therefore whoever eats this bread or drinks this cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord. 28 But let a man examine himself, and so let him eat of the bread and drink of the cup. 29 For he who eats and drinks in an unworthy manner eats and drinks judgment to himself, not discerning the Lords body. 30 For this reason many are weak and sick among you, and many sleep.