The phrase “not of this world” is perhaps most well-known for being a Christian apparel company (http://www.notw.com/). But what exactly does it mean that Christians are “not of this world”? The phrase comes from John 18:36 where Jesus says that His kingdom is “not of this world.” As His followers, Christians are members of His kingdom which is “not of this world,” that is, heaven (Philippians 3:20). Yes, we are on earth for now, but our earthly lives are nothing but a vapor (James 4:14). But eternity, now that is a long time, and that is where a Christian’s focus should really be (1 Peter 5:10).
The things of this world, wars, famine, suffering, poverty, etc., impact Christians and non-Christians alike. By remembering that we are “not of this world,” remembering that these things are just for a little while, we can see them in a different light. We are still in this world but we are no longer of it (John 17:14). We are still surrounded by all the horrors and tragedy of this life, but this is not our life (Philippians 3:8–14). The knowledge that we are not of this world gives Christians hope even in the darkest times (1 Peter 1:6–9); hope that this will pass and at the end of it we will be in heaven with our God, face to face forever (Revelation 21:3–4). This cracked and broken place is not where we belong, and it is not where we will stay (1 Corinthians 3:12).
Christians are not of this world. We have been adopted as heirs of heaven by God Himself and that is our world, our citizenship (Titus 3:7). And in the meanwhile we wait (2 Corinthians 5:12), and we hope (Romans 5:5), and we do what we can to bring others into the “not of this world” relationship with Jesus Christ. But this world is not our home, and never will be.
 Got Questions Ministries. (2010). Got Questions? Bible Questions Answered. Bellingham, WA: Logos Bible Software.