Biblical citations in the hymnal:
John 1:6-9, 15-17, 23-31;
Matthew 3:1-12; 11:9-14;
Luke 1:41-44
John 1:6-9: "There was a man sent from God, whose name was John. He came as a witness, to bear witness about the light, that all might believe through him. He was not the light, but came to bear witness about the light.
"The true light, which enlightens everyone, was coming into the world."
John 1:15-17: "(John bore witness about him, and cried out, 'This was he of whom I said, "He who comes after me ranks before me, because he was before me."') And from his fullness we have all received, grace upon grace. For the law was given through Moses; grace and truth came through Jesus Christ."
John 1:23-31: "He [John the Baptist] said, 'I am the voice of one crying out in the wilderness, "Make straight the way of the Lord," as the prophet Isaiah said.'
"(Now they ["priests and Levites from Jerusalem"] had been sent from the Pharisees.) They asked him, 'Then why are you baptizing, if you are neither the Christ, nor Elijah, nor the Prophet?' John answered them, 'I baptize with water, but among you stands one you do not know, even he who comes after me, the strap of whose sandal I am not worthy to untie.' These things took place in Bethany across the Jordan, where John was baptizing.
"The next day he saw Jesus coming toward him, and said, 'Behold, the Lamb of God, who takes away the sin of the world! This is he of whom I said, "After me comes a man who ranks before me, because he was before me." I myself did not know him, but for this purpose I came baptizing with water, that he might be revealed to Israel.'"
Matthew 3:1-12: "In those days John the Baptist came preaching in the wilderness of Judea, 'Repent, for the kingdom of heaven is at hand.' For this is he who was spoken of by the prophet Isaiah when he said, 'The voice of one crying in the wilderness: "Prepare the way of the Lord; make his paths straight."' Now John wore a garment of camel's hair and a leather belt around his waist, and his food was locusts and wild honey. Then Jerusalem and all Judea and all the region about the Jordan were going out to him, and they were baptized by him in the river Jordan, confessing their sins.
"But when he saw many of the Pharisees and Sadducees coming to his baptism, he said to them, 'You brood of vipers! Who warned you to flee from the wrath to come? Bear fruit in keeping with repentance. And do not presume to say to yourselves, 'We have Abraham as our father,' for I tell you, God is able from these stones to raise up children for Abraham. Even now the axe is laid to the root of the trees. Every tree therefore that does not bear good fruit is cut down and thrown into the fire.
"'I baptize you with water for repentance, but he who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire. His winnowing fork is in his hand, and he will clear his threshing floor and gather his wheat into the barn, but the chaff he will burn with unquenchable fire.'"
Matthew 11:9-14: "
'What then did you go out to see? A prophet? Yes, I tell you, and more than a prophet. This is he of whom it is written, "Behold, I send my messenger before your face, who will prepare your way before you." Truly, I say to you, among those born of women there has arisen no one greater than John the Baptist. Yet the one who is least in the kingdom of heaven is greater than he. From the days of John the Baptist until now the kingdom of heaven has suffered violence, and the violent take it by force. For all the Prophets and the Law prophesied until John, and if you are willing to accept it, he is Elijah who is to come.'"
Luke 1:41-44: "And when Elizabeth heard the greeting of Mary, the baby leaped in her womb. And Elizabeth was filled with the Holy Spirit, and she exclaimed with a loud cry, 'Blessed are you among women, and blessed is the fruit of your womb! And why is this granted to me that the mother of my Lord should come to me? For behold, when the sound of your greeting came to my ears, the baby in my womb leaped for joy.'"
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The first verse comes from the John 1 readings. The first lines ("When all the world was cursed / By Moses' condemnation") refer to "For the law was given through Moses," and the lines "With true forerunner's zeal / The greater One he [John the Baptist] named" come from John's proclaiming that - as the Matthew account phrases it - "he who is coming after me is mightier than I."
The beginning of the second verse ("Before he [John the Baptist] yet was born, / He leaped in joyful meeting" comes from the Luke 1 text. "By Jordan's rolling stream, / A new Elijah bold" seems to refer to the Matthew 11 text, where Jesus says, "if you are willing to accept it, he is Elijah who is to come."
The third verse has the phrase "Behold the Lamb of God" twice, which comes from the later part of the John 1 text. Once, it's followed by "That bears the world's transgression," and the other time, by "Who takes away our sin," both of which are paraphrases of John's "who takes away the sin of the world."
The last verse - specifically "That gladly we may walk / Upon our Savior's way / Until we live with Him / In His eternal day" - seems to be from the Matthew 3 text, just without the metaphorical language of "he will clear his threshing floor and gather his wheat into the barn."
There are various other phrases in the hymn that describe John's proclaiming Jesus as the Savior ("And Him... As Savior he proclaimed," "Confessing Him as Lord," "He testified of Him"), but they could really come from any of the texts cited.
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While transcribing this, I noticed that there's structural parallelism between "That bears the world's transgression" and "Who takes away our sin" in verse 3, but I think the perspective here is significant. Both speak of Christ's redemption, but while the first is global, the second is more personal, using a first person possessive adjective ("our").