As no one was able to enter the temple (Rev.15:8), the loud voice coming from it must be the voice of God himself: the time is come, and nothing can now prevent his wrath upon humanity.
The calamities following the pouring out of each bowl are similar in nature to those resulting from the blowing of the seven trumpets, they follow the same sequence, and four of them had already been applied to Egypt in antiquity. Their severity, however, is much greater, and they hit all of rebellious mankind: their purpose is to inflict a complete and final punishment, and it seems that the bowls are poured in rapid succession (Matt. 24:22).
The pouring of the first bowl directly impacts those who carry the mark of the Beast and worship his image: they will be afflicted by some kind of skin ulcer or malignancy (Exo.9:9-11; Deut.28:35).
The second trumpet judgment struck one third of the salt water, but when the second bowl is poured, all the rest of the salt water will be turned into blood, destroying the remainder of sea life.
Similarly, one third of fresh water was made poisonous at the sounding of the third trumpet, but the rest of all fresh water in rivers and fountains is turned into blood when the third bowl is poured (Exo. 7:17-25, Psa. 78:44 - will water in wells and cisterns also be contaminated?).
Two declarations are made:
by the angel of the waters, to the effect that the righteousness of the Lord is revealed in this judgment, as these people killed saints and prophets.
from the altar, confirming the righteousness of the judgment of the Lord God Almighty. The voice is probably coming from the souls of the tribulation martyrs, satisfied with God's vengeance (Rev.6:9-11), in which case the number of the faithful who will die for their faith will by then have been completed.
The fourth trumpet judgment resulted in a reduction by one third of the light of the sun, moon, and stars, whereas upon the pouring out of the fourth bowl the heat of the sun is increased to a point where men are scorched. Mankind will recognise that this punishment is coming from God, but instead of repenting and turning to Him for mercy, they will blaspheme His name.
The fifth trumpet judgment brought about the third blackout, among other things. The fifth bowl judgment causes the fourth blackout. The previous blackouts will have been: before the tribulation (Joel 2:31), on the opening of the sixth seal (Rev. 6:12) and on the sounding of the fifth trumpet (Rev. 9:2). There will be a fifth blackout immediately after the tribulation (Matt. 24:29, Joel 3:14-17). The entire kingdom of the beast will be darkened, which encompasses all the world with the exception of the three Transjordanian nations (Exo.10:21-23, Dan.11:41). Its inhabitants will suffer a gnawing pain (sunburn, disease, ulcers will be contributing factors), which will cause them to blaspheme all the more.
The sixth bowl judgment marks the beginning of the Campaign of Armageddon: the Euphrates river will be dried up to expedite the gathering and manoeuvring of the Babylonian forces from the East, presently Iraq, to which will probably be added Syria, Iran and Turkey. There will be a summons from the satanic trinity (the dragon, the beast, and the false prophet), reinforced by demonic activity, to make sure that the nations of the world, under seven kings, will indeed cooperate in assembling their armies in Armageddon. Armageddon is a combination of two Hebrew words which translate as the mountain of Megiddo. Meggido was a strategic city located at the western end of the Valley of Jezreel, guarding the famous Meggido Pass into Israel's largest valley. So what is commonly known as the Valley of Armageddon is actually the biblical Valley of Jezreel. There they will assemble for the battle on the great day of God, the Almighty: the Jews refuse to submit to the Antichrist's world authority, and by seeking to destroy them, Satan also seeks to break any remaining control God has over mankind (Psa2:1-6). The campaign of Armageddon culminates a series of crusades involving various battles, some in Egypt (Dan.11:40-45), and finally Jerusalem (Zech.14:2).
In verse 15 we have another digression, addressed by the Lord Jesus to the faithful living at that time, including the remnant Jews who have taken refuge in the mountains: it reminds them of the teaching in Matthew 24 concerning the end of the period of tribulation, when He comes again; they are encouraged to continue firm in the faith, for when they see the gathering of the armies together, then they can know that His second coming is imminent; they are warned to be prepared for Him so as not be made ashamed (nude means not ready - Luke 12:35-48).
The seventh bowl is poured into the air, the last act of the wrath of God in this series, resulting in tremendous disasters: the greatest earthquake ever will take place, destroying mountains and islands, as well as the cities of the nations. The great city, Jerusalem (Zech.14:4 and Rev.11:8), will be divided in three parts, and Babylon will be destroyed under God's fury. Earth will be bombarded with hailstones, each weighing between 20 and 50 kilos.
Even after this, men will again blaspheme God because of their punishment.
1 Then I heard a loud voice from the temple saying to the seven angels, "Go and pour out the bowls of the wrath of God on the earth."
First Bowl: Loathsome Sores
2 So the first went and poured out his bowl upon the earth, and a foul and loathsome sore came upon the men who had the mark of the beast and those who worshiped his image.
Second Bowl: The Sea Turns to Blood
3 Then the second angel poured out his bowl on the sea, and it became blood as of a dead man; and every living creature in the sea died.
Third Bowl: The Waters Turn to Blood
4 Then the third angel poured out his bowl on the rivers and springs of water, and they became blood.
5 And I heard the angel of the waters saying: "You are righteous, O Lord, The One who is and who was and who is to be, Because You have judged these things.
6 For they have shed the blood of saints and prophets, And You have given them blood to drink. For it is their just due."
7 And I heard another from the altar saying, "Even so, Lord God Almighty, true and righteous are Your judgements."
Fourth Bowl: Men Are Scorched
8 Then the fourth angel poured out his bowl on the sun, and power was given to him to scorch men with fire.
9 And men were scorched with great heat, and they blasphemed the name of God who has power over these plagues; and they did not repent and give Him glory.
Fifth Bowl: Darkness and Pain
10 Then the fifth angel poured out his bowl on the throne of the beast, and his kingdom became full of darkness; and they gnawed their tongues because of the pain.
11 They blasphemed the God of heaven because of their pains and their sores, and did not repent of their deeds.
Sixth Bowl: Euphrates Dried Up
12 Then the sixth angel poured out his bowl on the great river Euphrates, and its water was dried up, so that the way of the kings from the east might be prepared.
13 And I saw three unclean spirits like frogs coming out of the mouth of the dragon, out of the mouth of the beast, and out of the mouth of the false prophet.
14 For they are spirits of demons, performing signs, which go out to the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.
15 "Behold, I am coming as a thief. Blessed is he who watches, and keeps his garments, lest he walk naked and they see his shame."
16 And they gathered them together to the place called in Hebrew, Armageddon.
Seventh Bowl: The Earth Utterly Shaken
17 Then the seventh angel poured out his bowl into the air, and a loud voice came out of the temple of heaven, from the throne, saying, "It is done!"
18 And there were noises and thunderings and lightnings; and there was a great earthquake, such a mighty and great earthquake as had not occurred since men were on the earth.
19 Now the great city was divided into three parts, and the cities of the nations fell. And great Babylon was remembered before God, to give her the cup of the wine of the fierceness of His wrath.
20 Then every island fled away, and the mountains were not found.
21 And great hail from heaven fell upon men, each hailstone about the weight of a talent. Men blasphemed God because of the plague of the hail, since that plague was exceedingly great.
Rev 16:1-21 (NKJV)