When Jesus said, “it is finished” He didn’t mean our part in this battle, but rather His part of the battle.
Jesus came to earth to redeem mankind from the power of Satan.
Hebrews 2: 14-16 Since then the children share in flesh and blood, He Himself likewise also partook of the same, that through death He might render powerless him who had the power of death, that is, the devil; and might deliver those who through fear of death were subject to slavery all their lives. For assuredly He does not give help to angels, but He gives help to the descendant of Abraham.
Before He came we had no chance to defeat him and prevail over him for the reclaiming of the earth.
Thus God sustained mankind through the Covenant of promise and the Covenant of Law until the redeemer would come.
What then was “finished” when He said, “it is finished.” It might be good to define exactly what was finished, because what wasn’t finished is what we have to finish.
Though Satan was defeated positionally and definitely he was not defeated experiencially.
Essentially Satan was stripped of His power to hold us and have dominion over us, but only through the “faith application” of those who are in the Lord Jesus Christ. The rest of earth is under his power and deceit and shall be unless we rise up against Him.
The final defeat of Satan and the Curse is being brought about by the believers of the Lord Jesus Christ (his body who fills all in all).
When Jesus defeated Satan he was cast down to the earth.
He came to the earth with a great vengeance to make war with the children of God.
Revelation 12:9 And the great dragon was thrown down, the serpent of old who is called the devil and Satan, who deceives the whole world; he was thrown down to the earth, and his angels were thrown down with him. NASB
He will continue to have a realm of dominion in the earth until the saints rise up against him and subdue his works in the earth.
Christ is not just returning to conquer the curse and the last enemy (death) He is returning because the Church will have become a glorious triumphant bride for the Son of God.
Even though Satan is defeated positionally, he has been granted an extension of rule (including all his spiritual powers) until we subject him to the dominion of Jesus Christ.
His rule is relative to the sons of men who have not received Jesus and begun to exercise authority over Him.
We are called and empowered to subject Satan to the Kingdom of Christ.
We must subjecting him to Christ and dispossessing him.
Then we also are defeating him and his influence in the earth by bringing more people to Christ and teaching them to rule over him.
People are the real estate that this battle is all about. The more people turn to Jesus the more we are occupying his land and plundering his evil.
This is partially why you don’t want to play with evil, because it promotes his agenda and his power over the earth.
Sin is what gives him legal power over a people and nation.
Therefore he loves all the sin we indulge in! It is his guarantee of power and extension to his kingdom. The kingdoms work by displacement.