First of all, I wouldn't call the few verses that talk about a new heaven and a new Earth "the message of Christianity," and secondly, no, you didn't understand it correctly.
God's not going to build a giant space ark to take all the righteous people to another planet while He destroys the Earth because it's ugly. As any astrophysicist knows, the Earth and the Sun and this solar system are all doomed anyway--even if you don't believe in God, we know the Earth will one day be destroyed as it is swallowed up in the fire of an expanding Sun.
Hey, that sounds an awful lot like 2 Peter 3:7&10...
"By the same word the present heavens and earth are reserved for fire, being kept for the day of judgment and destruction of the ungodly."
"But the day of the Lord will come like a thief. The heavens will disappear with a roar; the elements will be destroyed by fire, and the earth and everything done in it will be laid bare."
When John saw "a new heaven and a new earth" in Revelation 21:1-4, he wasn't seeing a new physical planet, he was seeing a new environment where God dwells with us--a new ground that wasn't the familiar ground (and there was no sea) and a new sky that wasn't the familiar sky--in short, what we typically call "heaven."
"Then I saw “a new heaven and a new earth, for the first heaven and the first earth had passed away, and there was no longer any sea. I saw the Holy City, the new Jerusalem, coming down out of heaven from God, prepared as a bride beautifully dressed for her husband. And I heard a loud voice from the throne saying, 'Look! God’s dwelling place is now among the people, and he will dwell with them. They will be his people, and God himself will be with them and be their God. He will wipe every tear from their eyes. There will be no more death or mourning or crying or pain, for the old order of things has passed away.'"