Instruments regarding the Messiah
The people had been in mourning during the time of the Babylonian captivity. Jeremiah prophesies that the Jews will return from exile and that the coming of the Messiah will result in the restoration of instrumental music —
(Jer 31:2-13 ESV) 2 Thus says the LORD: “The people who survived the sword found grace in the wilderness; when Israel sought for rest, 3 the LORD appeared to him from far away. I have loved you with an everlasting love; therefore I have continued my faithfulness to you. 4 Again I will build you, and you shall be built, O virgin Israel! Again you shall adorn yourself with tambourines and shall go forth in the dance of the merrymakers. … 7 For thus says the LORD: “Sing aloud with gladness for Jacob, and raise shouts for the chief of the nations; proclaim, give praise, and say, ‘O LORD, save your people, the remnant of Israel.’ …
12 They shall come and sing aloud on the height of Zion, and they shall be radiant over the goodness of the LORD, over the grain, the wine, and the oil, and over the young of the flock and the herd; their life shall be like a watered garden, and they shall languish no more. 13 Then shall the young women rejoice in the dance, and the young men and the old shall be merry. I will turn their mourning into joy; I will comfort them, and give them gladness for sorrow.
This is unquestionably a celebration of the Messiah and the coming of the Kingdom, as shown by the next few verses, quoted in Hebrews 8 as speaking of the new covenant —
(Jer 31:31-34 ESV) 31 “Behold, the days are coming, declares the LORD, when I will make a new covenant with the house of Israel and the house of Judah, 32 not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the LORD. 33 But this is the covenant that I will make with the house of Israel after those days, declares the LORD: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people. 34 And no longer shall each one teach his neighbor and each his brother, saying, ‘Know the LORD,’ for they shall all know me, from the least of them to the greatest, declares the LORD. For I will forgive their iniquity, and I will remember their sin no more.”
While Zechariah doesn’t mention instruments, he clearly associates singing with the coming of the Messiah. Singing, like musical instruments, is about God’s approval and celebration.
(Zec 2:9-11 ESV) 9 “Behold, I will shake my hand over them, and they shall become plunder for those who served them. Then you will know that the LORD of hosts has sent me. 10 Sing and rejoice, O daughter of Zion, for behold, I come and I will dwell in your midst, declares the LORD. 11 And many nations shall join themselves to the LORD in that day, and shall be my people. And I will dwell in your midst, and you shall know that the LORD of hosts has sent me to you.
Instruments regarding the end times
When Isaiah prophesies God’s destruction of the earth at the end of time (see John N. Oswalt, The New International Commentary on the Book of Isaiah Chapters 1 – 39, pp 444 ff), he describes it in terms of the end of instrumental music as punishment for violating God’s “everlasting covenant.”
(Isa 24:7-10 ESV) 7 The wine mourns, the vine languishes, all the merry-hearted sigh. 8 The mirth of the tambourines is stilled, the noise of the jubilant has ceased, the mirth of the lyre is stilled. 9 No more do they drink wine with singing; strong drink is bitter to those who drink it. 10 The wasted city is broken down; every house is shut up so that none can enter.
Therefore, instrumental music has eschatological significance, symbolizing God’s pleasure, while the absence of instrumental music indicates his displeasure.
Of course, the coming of the Messiah and the coming of the Kingdom will be greatest possible cause of celebration. Therefore, the mourning that accompanied the Exile will be replaced with celebration and instrumental music.
When God ends the Messianic age and begins the eternal age, he’ll destroy the wicked and they’ll no longer enjoy their instrumental music. But the saved will be ushered into bliss with Jesus, where they’ll celebrate and worship with instruments, as is appropriate.
(Rev 14:2-3 ESV) 2 And I heard a voice from heaven like the roar of many waters and like the sound of loud thunder. The voice I heard was like the sound of harpists playing on their harps, 3 and they were singing a new song before the throne and before the four living creatures and before the elders. No one could learn that song except the 144,000 who had been redeemed from the earth.
(Rev 15:2-4 ESV) 2 And I saw what appeared to be a sea of glass mingled with fire–and also those who had conquered the beast and its image and the number of its name, standing beside the sea of glass with harps of God in their hands. 3 And they sing the song of Moses, the servant of God, and the song of the Lamb, saying, “Great and amazing are your deeds, O Lord God the Almighty! Just and true are your ways, O King of the nations! 4 Who will not fear, O Lord, and glorify your name? For you alone are holy. All nations will come and worship you, for your righteous acts have been revealed.”
You see, it’s a mistake to dismiss the harps in the Revelation as “symbolic” and hence meaningless or being a part of a different age. The question isn’t whether the harps are symbolic. Of course, they are. They question is why John chose to use harps as symbols.
And the answer is that instrumental music has been a part of God’s worship, not only in the Temple, but in other times and places going back for millennia. The scriptures have uniformly associated instrumental music with God’s pleasure, with worship, and with celebration. And the scriptures have uniformly associated the absence of instruments with God’s displeasure and mourning. That’s why the symbols are chosen.
And what the use of those symbols tells us is that God, inspiring John, hasn’t changed his mind. Those associations make the symbols meaningful to readers of his New Testament book.