The Christians have nothing to fear from this coming judgment. At this juncture God's warnings are exhausted, there remains now only the execution of His wrath. God's mercy always tempers His wrath, and that is why only one-third of man and creation have fallen under judgment so far. But now, for Rome, the demarcation between God's mercy and wrath has overlapped, He now reveals to John that His wrath will no long be mixed with mercy. John sees the obvious, God is prepared to pour out His full wrath on unrepentant Rome.
Fire is always indicative of judgment.
God wants John to understand that His mercy is extended to the limits. Pictured for John are the victorious Saints who will now see the righteous indignation of God against their persecutors of earth. John records that the Redeemed of the Ages respond by singing praises to God, as is typical in Heaven. John is viewing another one of those preludes to victory designed to comfort the Christians who would witness the awesome wrath of God being poured out on Rome. John is to show those Faithful Ones still on earth this picture of their Brethren in Heaven.