And I heard a great voice saying in the heaven, `Now did come the salvation, and the power, and the reign, of our God, and the authority of His Christ, because cast down was the accuser of our brethren, who is accusing them before our God day and night;
Revelation 12:8 and they did not prevail, nor was their place found any more in the heaven;
Revelation 12:9 and the great dragon was cast forth--the old serpent, who is called `Devil,' and `the Adversary,' who is leading astray the whole world--he was cast forth to the earth, and his messengers were cast forth with him.
Revelation 12:10 And I heard a great voice saying in the heaven, `Now did come the salvation, and the power, and the reign, of our God, and the authority of His Christ, because cast down was the accuser of our brethren, who is accusing them before our God day and night;
Revelation 12:11 and they did overcome him because of the blood of the Lamb, and because of the word of their testimony, and they did not love their life--unto death;
Revelation 12:12 because of this be glad, ye heavens, and those in them who do tabernacle; woe to those inhabiting the land and the sea, because the Devil did go down unto you, having great wrath, having known that he hath little time.'
The verse centers on "heard", "great", "voice", "saying", "heaven", "come", "salvation", and "power". It is saying that the verse draws attention to "heard" and "great", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 9's "and the great dragon was cast forth--the..." into verse 11's "and they did overcome him because of...", so "heard" and "great" belong inside that flow. In Revelation context, the local focus is Christ, faith, and discipleship.
A plain takeaway is to answer the verse's own emphasis on "heard" and "great" with trust shaped by these words, not by a vague optimism outside the passage.