Try to imagine this. It is the year 1944 and Germany is at war with England.
Now suppose you have dual citizenship in both Germany and England and you are living in Germany. How will you be treated?
Conversely, suppose you have dual citizenship in both Germany and England and you are living in England during the Blitz; how would you be treated?
In either case, you would be forced to renounce your citizenship in the opposing country and declare your loyalty to the country in which you found yourself in that dreadful time. There would be no middle ground.
They would have never allowed dual citizenship in Germany and England during WWII and we cannot have dual citizenship in this world. We cannot belong to the Kingdom of God by living in Christ AND live in the kingdom of darkness at the same time. They are diametrically and violently opposed to one another!
The spiritual war going on for the hearts of men and women in this world is no less intense than the war that took place between the Axis and the Allies in WWII!
The only difference would be the tactics.
The evil one has us by the throat because of sin. He wishes to destroy us not only in this life but in Hell forever where we will find ourselves with him and the rest of his horde.
God, on the other hand, loves us so much that He sent His one and only Son to die on the cross of Calvary to make a way for our sins to be forgiven, to adopt us into His family and to give us eternal life.