Why is the believer's citizenship in Heaven according to the Body of Christ analogy?

The believer's citizenship is in Heaven because every true believer is a member of the Body of Christ, whose Head is in Heaven. As the physical body is controlled by its head, so the Body's nerve center and Head are in Heaven, making Heaven the place of the believer's citizenship.

Les teaches from Ephesians 2 that the Body of Christ is like a human body: one body with many members, but the control is at the head. Since believers are now members of the Head, who is in Heaven, that is where their citizenship is—where the head and nerve center are. He adds that this is a Pauline term and revelation, not found in the Gospels or with Peter, and that by one Spirit all true believers—not merely the most spiritual or those who had an emotional upheaval—are baptized into the Body of Christ.

1 Corinthians 12:12

For as the body is one, and hath many members, and all the members of that one body, being many, are one body: so also is Christ.

Where is the head of the Body of Christ, and what does that mean for believers?

The head of the Body of Christ is in Heaven, where the Head and nerve center are. Believers are members of that Head, so their citizenship is in Heaven. By one Spirit, the Holy Spirit baptizes every true believer into the Body of Christ, not just the most spiritual.