Matthew 3:13-16: "13 Then Jesus came from Galilee to the Jordan to John, to be baptized by him. 14 John would have prevented him, saying, 'I need to be baptized by you, and do you come to me?' 15 But Jesus answered him, 'Let it be so now, for this it is fitting for us to fulfill all righteousness.' Then he consented. 16 And when Jesus was baptized, immediately he went up from the water, and behold, the heavens were opened to him, and he saw the Spirit of God descending like a dove and coming to rest on him"
1 Peter 2:9: "But you are a chosen race, a royal priesthood, a holy nation, a people for his own possession, that you may proclaim the excellencies of him who called you out of darkness into his marvelous light."
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The text from Matthew is the basis for the first verse. Matthew 3:17 should also be included: "and behold, a voice from heaven said, 'This is my beloved Son, with whom I am well pleased.'" In the hymn, this is rendered as "Hear how the voice from heaven thunders, / 'Lo, this is My beloved Son.'" The first part of Matthew 4 is also summarized in the beginning of the second verse: "From this assurance of God's favor / Jesus goes to the wilderness, / There to endure a time of testing."
1 Peter 2:9 appears in the third verse, specifically in the lines "Christ's royal priesthood help us share" and "Raise up in us a chosen people."