We had met the term “seven” in relation to “seven spirits of God,” “seven churches,” “seven seals,” etc. The term denoted the concept of fullness, completeness. “Seven thunders” signifies a completeness of thunder, overwhelming thunder. At the exodus from Egypt, God thundered so mightily as he opened up the Red Sea to deliver his people that “the earth trembled and shook” (Psalm 77:17–18). At Mount Sinai the Lord revealed his majesty through thunder, so that in turn the Israelites at the foot of the mountain trembled (Exodus 19:16, Exodus 19:19; Exodus 20:18–19). Here we have an overpowering display of God’s majesty as he acts for the salvation of his people (and the destruction of his enemies).
3 and called out with a loud voice, like a lion roaring. When he called out, the seven thunders sounded.