8 “The lion has roared” sounds an alarm. There is indeed cause for fear, though not from any lion or blast of a trumpet; it is Yahweh’s voice through his prophet that should strike fear in people’s hearts. Yahweh is no longer stalking quietly (cf. v. 4)—he has pounced! He has spoken, and no one can contravene his word. So Amos pronounces judgment on the people. For a possible connection with 1:2, see comments on 1:2.
The Expositor’s Bible Commentary
3:8 who can but prophesy. Just as a lion’s voice evokes fear, so the voice of the Lord compels the prophets to proclaim His word (Deut. 18:18; cf. 1 Cor. 9:16).
The Reformation Study Bible