Acts 1:10-11: "10 And while they were gazing into heaven as he went, behold, two men stood by them in white robes, 11 and said, 'Men of Galilee, why do you stand looking into heaven? This Jesus, who was taken up from you into heaven, will come in the same way as you saw him go into heaven.'"
1 Thessalonians 4:16-17: "16 For the Lord himself will descend from heaven with a cry of command, with the voice of an archangel, and with the sound of the trumpet of God. And the dead in Christ will rise first. 17 Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air, and so we will always be with the Lord."
1 Corinthians 11:23-26: "23 For I received from the Lord what I also delivered to you, that the Lord Jesus on the night when he was betrayed took bread, 24 and when he had given thanks, he broke it, and said, 'This is my body which is for you. Do this in remembrance of me.' 25 In the same way also he took the cup, 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."
Luke 2:8-15: "8 And in the same region there were shepherds out in the field, keeping watch over their flock by night. 9 And an angel of the Lord appeared to them, and the glory of the Lord shone around them, and they were filled with fear. 10 And the angel said to them, 'Fear not, for behold, I bring you good news of great joy that will be for all the people. 11 For unto you is born this day in the city of David a Savior, who is Christ the Lord. 12 And this will be a sign for you: you will find a baby wrapped in swaddling cloths and lying in a manger.' 13 And suddenly there was with the angels a multitude of the heavenly host praising God and saying, 14 'Glory to God in the highest, and on earth peace among those with whom he is pleased!'
"15 When the angels went away from them into heaven, the shepherds said to one another, 'Let us go over to Bethlehem and see this thing that has happened, which the Lord has made known to us.'"
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The text is public domain:
Let all mortal flesh keep silenceAnd with fear and trembling stand;Ponder nothing earthly minded,For with blessing in His handChrist our God to earth descendingComes our homage to demand.
King of kings yet born of Mary,As of old on earth He stood,Lord of lords in human vesture,In the body and the blood,He will give to all the faithfulHis own self for heav'nly food.
Rank on rank the host of heavenSpreads its vanguard on the wayAs the Light of Light, descendingFrom the realms of endless day,Comes the pow'rs of hell to vanquishAs the darkness clears away.
At His feet the six-winged seraph,Cherubim with sleepless eye,Veil their faces to the presenceAs with ceaseless voice they cry:"Alleluia, alleluia!Alleluia, Lord Most High!"
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The title line seems to be taken from Zechariah 2:13: "Be silent, all flesh, before the LORD, for he has roused himself from his holy dwelling."
The first and third verses seem to come from 1 Thessalonians 4:16-17 and possibly also Acts 1:10-11 ("'Jesus... will come in the same way as you saw him go into heaven'"). The description "with fear and trembling" in the first verse seems to be drawn from Luke 2:9 (the shepherds "were filled with fear").
The first half of the second verse comes from Luke 2:8-15, and the second half comes from 1 Corinthians 11:23-26. The titles "King of kings" and "Lord of lords" are from Revelation 19:16: "On his robe and on his thigh he has a name written, King of kings and Lord of lords."
The fourth verse seems to refer to Isaiah 6:2 ("Above him stood the seraphim. Each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew."), although what the angels are saying is more closely related to Luke 2:14.
