Revelation 10:3

And when he cried out, the seven thunders uttered their voices. When the Lord preaches mightily, the Church, also full of sevenfold grace, raises its voices to preach. For the lion roars, who will not fear? The Lord God has spoken, who will not prophesy (Amos III)? For the seven thunders are the same as the seven trumpets of which he speaks.

[The loud voice] indicates that through his servants he was proclaimed faithfully and with strength. As the prophet says, "And he made firm the glory of his strength." "When he called out, the seven thunders sounded." Because of the known use of the number seven, I think that the seven thunders signify the various manners of the church's preaching. The apostle Paul spoke of this concerning his service to others, "As babes in Christ, I fed you with milk, not solid food," however to others, "solid food is for the perfect," yet also to others, "avoid the heretical man after the first and second admonition." Therefore, the church is said to proclaim rightly through its usual offices of preaching.

And he cried out with a loud voice, as when a lion roars. And it is said that the holy angel of wrath was symbolically represented as having roared like a lion, signifying the judgment against the impious. And when he said, "he cried out," the seven thunders uttered their voices. The seven thunders refer to the seven ministering spirits whose memory has been mentioned earlier; therefore, it is placed after the article. He says, "the seven thunders," referring to those seven spirits to which the reference pertains. What then is the meaning of the seven spirits crying out? It is that they also fully correspond with the punishments inflicted upon sinners, offering a grateful hymn of praise to God, acknowledging that all things have been done justly. At the same time, the one who cried out clarified the various forms of punishments.