Ephesians 6:10-24.

Outside of Christ, the Ephesians were citizens of Satan's dark kingdom; his followers in the domain of darkness. But then came the gospel of salvation in Christ alone. One result of repenting of sin and believing this gospel was a kingdom transfer. In Christ Jesus, the Ephesians were now citizens of His kingdom, "delivered from the domain of darkness and transferred to the kingdom of [God's] beloved Son" (Col. 1:13). And now as recipients of God's grace, they are the opposition, adversaries of Satan's dark kingdom. Paul explains this is why life is a battle, and why we need to stand strong in the Lord's strength.