The letter to the Romans is the gospel written to believers. Why did believers need to hear the gospel? The first half of the letter, Chapters 1-8 contain the gospel of grace. The other half of this letter, Chapters 9-16 contain the gospel of peace.
In this latter half, the Apostle Paul urges believers to not feel superior over one another, whether Jew or Gentile, strong or weak in faith. He stresses over and over the importance of bearing with one another in love and accepting one another as Christ has accepted us.
In the final chapter Paul gives a homework assignment: He commands the believers in Rome to venture out of the group they are meeting with to greet other believers. He urges them to not get comfortable in their group with the folks who are most likely similar to them, and to “cross pollinate” with other believers who may have different perspectives than them, to express love and acceptance to them and spend time getting to know them. (This is the meaning of the particular Greek word Paul used for “greet” in his command to greet.)
Then (also in chapter 16) he gives a warning to stay away from those who cause divisions (verse 17). It seems Paul takes this topic of unity in the body of Christ very seriously.
After these instructions, the Apostle says, “The God of peace will soon crush satan under your feet.” (verse 20). It appears this sentence, the crushing of satan, cannot be fully carried out until the body of Christ has the revelation that we are members of one another in Christ and begin to speak of this to one another and treat each other accordingly.
This will fulfill the prophecy in Genesis 3:15 where the Seed of Eve will crush satan’s head. This was accomplished by Jesus Christ on the cross and is also carried out by members of His body seeing we are one in Christ. (God crushes satan under OUR feet.)
It appears that God has already defeated satan. But the devil has blinded believers to the reality of our oneness in Christ.
When the blinders come off, satan is crushed.