Colossians tells us that God has, “rescued us from the domain of darkness, and transferred us to the kingdom of His beloved Son.” Being trapped in this flesh here in this fallen world makes that a little hard to believe, doesn’t it? The truth is Christ’s kingdom is much more than that glorious place we go once we die. His kingdom is here among us—a kingdom of awesome rewards and great responsibilities.
As a member of God’s kingdom we are now His children, and as such we have also become slaves of righteousness, as Romans 6 tells us. The citizens of the kingdom of darkness are those who rebelliously practice sin. They spurn what they know of God, and are by nature slaves of sin. The bottom line is that all people are either slaves of sin or slaves of righteousness—there is no middle ground between these two kingdoms.
From Free from the Power of Sin: the Keys to Growing in God in Spite of Yourself