Well here's the actual quote from the book of Psalms.
“The righteous themselves will possess the earth, and they will [live] forever upon it.”—Psalm 37:29.

Now what it doesn't explain is how all of the righteous people are going to fit on the earth. God told us that he would raise the dead and they will be judged to see if they are willing to accept the word of God. If they are, then they, too will be saved and live on the earth as well.

My question is, will God expand the earth in some way to ensure that everyone fits on it? I suppose, on this, we have to have faith x