What does Revelation 10:3 mean?
Prior verses revealed a "mighty angel" straddling the border between the land and the sea. This verse reports that the mighty angel calls out with a loud voice. His voice sounds like a lion roaring. This comparison is another reason some commentators think this being is Christ. Revelation 5:5 identifies Jesus as the Lion of the tribe of Judah. Isaiah 42:13 says that "the LORD goes out like a mighty man, like a man of war he stirs up his zeal; he cries out, he shouts aloud, he shows himself mighty against his foes."This verse also describes "seven thunders" sounding when the mighty angel called out. We cannot positively identify the seven thunders, although they may be the same thunders that sounded at the end of the seal judgments (Revelation 8:5). Speculation here is unnecessary—the following verse indicates that the meaning of this event has been explicitly kept un-revealed. All we need to know about the seven thunders is the fact that they affirm the mighty angel's actions.
The number seven appears frequently in Revelation. There are seven lampstands, seven churches, seven seals, seven trumpets, seven thunders, and seven bowls. Seven is often portrayed as the number of perfection, and of God. This contrasts with six, the number of man and of imperfection.