Hebrews 9:11-28: "11 But when Christ appeared as a high priest of the good things that have come, then through the greater and more perfect tent (not made with hands, that is, not of this creation) 12 he entered once for all into the holy places, not by means of the blood of goats and calves but by means of his own blood, thus securing an eternal redemption. 13 For if the blood of goats and bulls, and the sprinkling of defiled persons with the ashes of a heifer, sanctify for the purification of the flesh, 14 how much more will the blood of Christ, who through the eternal Spirit offered himself without blemish to God, purify our conscience from dead works to serve the living God.
"15 Therefore he is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant. 16 For where a will is involved, the death of the one who made it must be established. 17 For a will takes effect only at death, since it is not in force as long as the one who made it is alive. 18 Therefore not even the first covenant was inaugurated without blood. 19 For when every commandment of the law had been declared by Moses to all the people, he took the blood of calves and goats, with water and scarlet wool and hyssop, and sprinkled both the book itself and all the people, 20 saying, 'This is the blood of the covenant that God commanded for you.' 21 And in the same way he sprinkled with the blood both the tent and all the vessels used in worship. 22 Indeed, under the law almost everything is purified with blood, and without the shedding of blood there is no forgiveness of sins.
"23 Thus it was necessary for the copies of the heavenly things to be purified with these rites, but the heavenly things themselves with better sacrifices than these. 24 For Christ has entered, not into holy places made with hands, which are copies of the true things, but into heaven itself, now to appear in the presence of God on our behalf. 25 Nor was it to offer himself repeatedly, as the high priest enters the holy places every year with blood not his own, 26 for then he would have had to suffer repeatedly since the foundation of the world. But as it is, he has appeared once for all at the end of the ages to put away sin by the sacrifice of himself. 27 And just as it is appointed for man to die once, and after that comes judgement, 28 so Christ, having been offered once to bear the sins of many, will appear a second time, not to deal with sin but to save those who are eagerly waiting for him."
John 1:29: "The next day he [John the Baptist] saw Jesus coming toward him, and said, ‘Behold, the Lamb of God, who takes away the sin of the world!"
John 6:51-58: "51 'I am the living bread that came down from heaven. If anyone eats of this bread, he 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, '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. 54 Whoever feeds on my flesh and drinks my blood has eternal life, and I will raise him up on the last day. 55 For my flesh is true food, and my blood is true drink. 56 Whoever feeds on my flesh and drinks my blood abides in me, and I in him. 57 As the living Father sent me, and I live because of the Father, so whoever feeds on me, he also will live because of me. 58 This is the bread that came down from heaven, not like the bread the fathers ate and died. Whoever feeds on this bread will live forever.'"
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The text from Hebrews is the main source for the hymn. It appears throughout but especially in the first four verses.
John 1:29 appears in the third, sixth, and seventh verses ("the holy Lamb," "The body of God's Lamb," and "Christ, God's Lamb").
The end of the fifth verse and all of the sixth come from the text from John 6.
The first part of the seventh verse ("With cherubim and seraphim / Our voices join the endless hymn, / And 'Holy, holy, holy' sing") draws from Isaiah 6:3: "And one [seraph] called to another and said: 'Holy, holy, holy is the LORD of hosts; the whole earth is full of his glory!'"
