This chapter describes Jesus sending messengers to Ethiopia with a warning and awaiting their response. It ends with them sending tribute to the temple in Zion. It is also a warning to Christians that the church will experience pruning in the end times.
This chapter is a warning to modern-day Ethiopia, the 'land of buzzing wings, the one beyond the rivers of Cush' (verse 1). Cush is modern-day Sudan, but is sometimes confused with Ethiopia because in classical times the region that now equates with southern Egypt and Sudan was named Aethiopia. Beyond that, from the geographical perspective of Israel, is modern-day Ethiopia. Verse 2 further describes it as a nation of tall, smooth-skinned people who are feared far and wide, a strong and victorious nation that is divided by rivers (v1-2). Daniel 11:43 tells us that Antichrist will control Egypt, and Cush will submit to him. Egypt and Sudan are both predominantly Muslim countries, whereas Ethiopia is predominantly Christian and may perhaps be able to resist him.

Verse 3 describes a banner being raised on the mountains that will be seen by all the people of the world, and a trumpet blast which is heard by everyone. This is surely the last trumpet. It coincides with Jesus' glorious second coming appearance (Matthew 24:31), the resurrection and rapture of believers (1 Thessalonians 4:16), and the audible angelic announcement to all who remain on earth that the kingdoms of this world have become the kingdom of our Lord and of his Christ (Revelation 11:15). It is time for every knee to bow and every tongue to confess Jesus as Lord (Philippians 2:10-11). This banner may also be a call for volunteers. Various scriptures imply that after his second coming, Jesus will muster an army to fight with him at the battle of Armageddon (search tags: Jesus musters an army).

Verse 2 starts with swift messengers being sent to Ethiopia. They have an urgent message to those who remain in Ethiopia. Jesus is watching and waiting, as though he has a particular spotlight focussed on Ethiopia (v4). What is he waiting for? He is waiting for Ethiopia to send important tribute that must be brought to God's temple in Zion (v7). What could this be? Verses 5 to 6 warn the Ethiopians that Jesus is going to prune Ethiopia, cutting off any unproductive shoots, and only leaving those that produce good fruit. Everyone must bow the knee to Jesus or face his judgment. But what is the tribute that Jesus specifically requires from Ethiopia?

One possible explanation is that the messengers are sent to request the Ark of the Covenant. It is almost universally believed by Ethiopians that the lost Ark of the Covenant from the Jerusalem temple is kept at the Church of Our Lady Mary of Zion in Axum Ethiopia. A problem with this explanation, however, is the prophecy in Jeremiah 3:14-18, which concerns the end-time restoration of Jerusalem as God's throne, and the reunification of Israel and Judah. Verse 16 of that prophecy says, "…At that time,” says the Lord, “people will no longer talk about having the ark that contains the Lord’s covenant with us. They will not call it to mind, remember it, or miss it. No, that will not be done any more!"

In verse 5, the harvest refers to the gathering and judgment of believers at the end of the age. Jesus repeatedly used this same metaphor (Matthew 9:37, 13:30, 21:41, 25:26, Mark 4:29, 12:2, Luke 10:2, 20:10, John 4:35). In his Parable of the Weeds in Matthew 13:24-29, Jesus foresaw that the church would become corrupt, and that at the end of the age the weeds (false believers) would first need to be uprooted and thrown into the fire before the wheat (true believers) could be gathered into the barn (at the rapture). Here in Isaiah 18, verse 5 is a warning that 'when the bud has sprouted', Jesus will allow the church to be pruned. In Matthew 24:32-33, Jesus said, "Learn this parable from the fig tree: Whenever its branch becomes tender and puts out its leaves, you know that summer is near. So also you, when you see all these things, know that he is near, right at the door." My conviction is that we are living today in the end-times when the bud has already started to sprout. We should therefore expect the church to be pruned in the years between now and the second coming. God allows believers to face trials as part of his pruning process, just as he allowed Peter to face trials on the night when Jesus was arrested (see Luke 22:31-34). Another harvest-type metaphor for end-time judgment is that of threshing (Jeremiah 51:33, Micah 4:12-13, Malachi 4:1, Luke 3:17).

Because the Ethiopian Orthodox church has such a long and proud Christian history, going back to the Ethiopian Eunuch in the first century AD (Acts 8:27), perhaps the Ethiopian church will need more pruning than some more modern church movements. Who knows? But all Christians and all churches should expect a degree of pruning in the end times. During the Great Tribulation, Christians must endure, and many will be martyred for their faith (Daniel 12:7, Revelation 2:10, 7:9-17, 12:11, 20:4-6).
The following video documents investigations by Bob Cornuke into the claim that the Ark of the Covenant is in Ethiopia.

The Mystery of the Ark of the Covenant - Bob Cornuke

Ethiopia had a large Jewish minority which was rescued and brought to Israel in the 1980's. No-one knows how or when Jews arrived in Ethiopia. It is possible they are one of the lost tribes from the northern kingdom of Israel which fell to Assyria in 722 BC. The following video documents their rescue by the Israelis:

Saving The Forgotten Jews - BBC News

Places: Ethiopia, Cush, Sudan, Axum
Symbols: End-time harvest, End-time pruning
Tags: Jesus sends messengers to Ethiopia, Cush submits to Antichrist, Jesus receives tribute from Ethiopia, Last trumpet, Jesus raises a banner, Every eye will see him, Every ear will hear, Nations bring tribute to Jesus, Ark of the Covenant, End-time harvest, End-time pruning
The Lord Will Judge a Distant Land in the South
18 Beware, land of buzzing wings, the one beyond the rivers of Cush,
2 that sends messengers by sea, who glide over the water’s surface in boats made of papyrus. Go, you swift messengers, to a nation of tall, smooth-skinned people, to a people that are feared far and wide, to a nation strong and victorious, whose land rivers divide.
3 All you who live in the world, who reside on the earth, you will see a signal flag raised on the mountains; you will hear a trumpet being blown.
4 For this is what the Lord has told me: “I will wait and watch from my place, like scorching heat produced by the sunlight, like a cloud of mist in the heat of harvest.”
5 For before the harvest, when the bud has sprouted, and the ripening fruit appears, he will cut off the unproductive shoots with pruning knives; he will prune the tendrils.
6 They will all be left for the birds of the hills and the wild animals; the birds will eat them during the summer, and all the wild animals will eat them during the winter.
7 At that time tribute will be brought to the Lord of Heaven’s Armies, by a people that are tall and smooth-skinned, a people that are feared far and wide, a nation strong and victorious, whose land rivers divide. The tribute will be brought to the place where the Lord of Heaven’s Armies has chosen to reside, on Mount Zion.