Ephesians opens with our spiritual blessings in heavenly places (Eph. 1:3), and it closes with our spiritual battle in high places (6:12).
Israel is God’s earthly people, and the Body of Christ is His heavenly people (Eph. 2:6). God gave Israel land and they had to fight flesh and blood enemies to possess and enjoy what was already theirs. Likewise, God has given us “all spiritual blessings in heavenly places” and we must fight spiritual enemies to appropriate and enjoy what is already ours. Just as Israel was to fight their enemies by depending on the power of God, we must be “strong in the Lord, and in the power of His might” if we are going to stand against our enemies.
In the book of Exodus, God redeemed Israel out of Egypt. In the book of Joshua, He brought them into the land of promise. In between those books (Numbers) we see that Israel failed to possess their land at the first because of unbelief and therefore wandered in the wilderness.
The book of Romans teaches us about our redemption by the blood of Christ. The book of Ephesians teaches us about the spiritual blessings God wants us to walk in and enjoy. In between those books we learn that it is possible for believers to not appropriate who they are in Christ because of unbelief and therefore wander in a spiritual wilderness (Corinthians and Galatians).