God wants to populate eternity only with people who have made a clear affirmative choice to commit to Him in love. If there was no opposing force operating in the world, it would be easy for everyone to choose God. But the devil tugs at people to choose something other than God, just as he did with Eve in the Garden of Eden. This does not mean people can't choose God, only that they must make a definitive choice in spite of all other alternatives. In this way God uses Satan help Him determine who will be granted eternal life. Once this process has been completed and everyone has made their choice, He will dispose of Satan once and for all.