Question:What is meant by a new heaven and new earth? And what is meant by a war in heaven?
"Now I saw a new heaven and a new earth, for the first heaven and the first earth had passed away. Also there was no more sea" (Revelation 21:1).
The first heaven and earth refers to where mankind currently dwells. This world is destined for destruction.
"But the day of the Lord will come as a thief in the night, in which the heavens will pass away with a great noise, and the elements will melt with fervent heat; both the earth and the works that are in it will be burned up. Therefore, since all these things will be dissolved, what manner of persons ought you to be in holy conduct and godliness, looking for and hastening the coming of the day of God, because of which the heavens will be dissolved, being on fire, and the elements will melt with fervent heat? Nevertheless we, according to His promise, look for new heavens and a new earth in which righteousness dwells" (II Peter 3:10-13).
Thus the new heaven and new earth is the replacement of the first. It is the new place God is preparing for His people to dwell. "In My Father's house are many mansions; if it were not so, I would have told you. I go to prepare a place for you. And if I go and prepare a place for you, I will come again and receive you to Myself; that where I am, there you may be also" (John 14:2-3). In other words, the new heaven and new earth is a reference to heaven.
"And war broke out in heaven: Michael and his angels fought with the dragon; and the dragon and his angels fought, but they did not prevail, nor was a place found for them in heaven any longer. So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him" (Revelation 12:7-9).
It means there was a conflict in heaven between those following Satan and those following God -- and Satan and his followers lost.