The short answer is nobody knows! The seven thunders appear in Revelation 10:1–7. In his apocalyptic vision, the apostle John saw a mighty angel striding the sea and the land with a scroll in his hand. The angel gave a shout like the roar of a lion, and then the voices of the seven thunders spoke. “And when the seven thunders had uttered their voices, I was about to write: and I heard a voice from heaven saying unto me, Seal up those things which the seven thunders uttered, and write them not.” Revelation 10:4
The incident of the seven thunderous voices occurs in the interlude between the sixth and seventh trumpets. The seven thunders are not just the usual noise of thunder but are thunder-like voices communicating a message. The Greek word translated “thunder” means “to roar.” Thunder is often a mark of judgment in Scripture as in 1 Samuel 2:10, 2 Samuel 22:14, and several more places in Revelation (8:5, 11:19, 16:18), so these seven powerful voices are crying out for God’s judgment upon the earth. The thunder represents the voice of God. “The Lord also thundered in the heavens, and the Highest gave his voice; hail stones and coals of fire.” Psalm 18:13
“And I saw another mighty Angel come down from heaven, clothed with a cloud: and a rainbow was upon his head, and his face was as it were the sun, and his feet as pillars of fire” Revelation 10:1
This mighty Angel is NOT an Angel at all it is Jesus Himself. The rainbow upon His head is symbolic of God’s covenant with man and the earth (Genesis 9:9-13). The significance of the bow’s association with Christ is that Jesus represents mankind. His sacrifice will never happen again, as Gods promise to never destroy the planet again with water will never happen again (Hebrews 10:11-14, 26).
The rainbow is type and shadow of Gods promise, which was associated with Christ as the promise of the redemption of man, which God made to Adam. This is depicted in the sacrifice of Cain and Able, Cain killed Able because God accepted Able’s sacrifice over Cain’s, the point of them giving sacrifices in the first place, was to show the sacrifice of the Messiah who was come.
This was instituted by God from the time of Adam after his sin, otherwise there is no way that Cain and Able would have ever known about sacrifices or their significance (Genesis 4:3-10).
Many references to Christ, describe Him in glory, or in an aspect of Him in judgment.
“Behold He commeth with clouds” (Rev 1:7).
“His face as it were the sun” (Rev. 10:1)
“and his face did shine as the sun” (Matthew 17:2)
“and His feet as pillars of fire” (Rev. 10:1)
The pillars of fire reference is equated with the reference, “and His feet like unto fine brass as if they burned in a furnace” (Rev. 1:15, 10:1, 19:11-13, 16, Daniel 7:9)
The reference to His face as the “Sun” is Christ’s unveiled glory.
His feet as “pillars” represent strength, the “fire” reference is associated with His judgment. All of man’s works will be judged by Christ, through fire (1 Cor. 3:13-15; 2 Cor. 5:10).
“And he had in his hand a little book open: and he set his right foot upon the sea, and his left foot on the earth” Revelation 10:2
The little book opened in Jesus’ hand is the book he took out of Gods right hand (Rev. 5:7). His feet on the earth and the sea, represent his right to dominion over the world.
“And cried with a loud voice, as when a lion roareth: and when he had cried, seven thunders uttered their voices” Revelation 10:3
The seven thunders cover the seven continents of the world. This means that when Jesus raised His voice, it was immediately followed with these quaking thunderous sounds heard all over the world.
“And when the seven thunders had uttered their voices”
These thunders are the voices of Angels introducing the last seven plagues that are to come in the tribulation, but John is not to write them down yet, not at this point.
“And out of the throne proceeded lightnings and thunderings and voices: and there were seven lamps of fire burning before the throne, which are the seven Spirits of God.”Revelation 4:5
The voice of God is depicted as thunderous rumblings that display the power, majesty and glory of the Lord. In this John saw a preview of the divine wrath to be poured out on the earth as described in full in Revelation chapters 16—19.
Just as John was about to write the words of the seven thunders, a voice from heaven commands him to seal up what was revealed by them. The same throne that issues forth lightning and peals of thunder issues a command to keep secret what the voices have revealed. The reason for the sealing is not given, but it could be that the judgment was simply too terrifying to be recorded. The content of the message is never revealed in Scripture, so we can’t speculate on it. The seven thunders are the only words in Revelation that are sealed.