Greet question! The concept of kingdom mostly has to do with God's rule. The kingdom of God would include all those who are subject to His authority. This certainly includes the church but it would also include, for example, angels. I tried to emphasize in this video that Christ is certainly ruling NOW. Thus, the kingdom is presently manifest in the world through the church. However, much of the world is still in rebellion against Christ, so Christ's conquest of the world is not yet complete. As Jesus taught us to pray, "your kingdom come, your will be done, on earth as it is in heaven" (Matthew 6:10). This prayer has not been fulfilled yet. In the resurrection, Christ's reign over the world will finally be complete/fulfilled. All of Christ's enemies (even death) will have been defeated: "Then the end will come, when he hands over the kingdom to God the Father after he has destroyed all dominion, authority and power. For he must reign until he has put all his enemies under his feet. The last enemy to be destroyed is death" (1 Corinthians 15:23-25). This is what I mean when I say the kingdom is not yet fulfilled. Some people will refer to this as the "already--not yet." The kingdom is already here and active in the world, but it is not yet what it shall be in the resurrection.