God has called all followers of Jesus to separate themselves from this world. We are in the world BUT not to be of it by nature. We have a new nature which is holy and righteous in which we are called to "walk" as Jesus did. If you flirt with and are friends with your old nature (the world) than you are "of it" and not being faithful to Jesus who gave you "His" new nature. Many are called to follow Jesus but not all prove faithful. Those who are faithful - love Him first, abide in His nature and they are chosen as His bride.