We don’t know when Paul wrote his letter to the Ephesians. His parting words to them in this final conversation with the elders included a warning against false teachers (Acts 20:28-30). Did this play on Paul’s mind?
After reminding them they were saved by a gift rather than by anything they had done, Paul gave some instructions for living together as believers:
Ephesians 4:1-3 I, therefore, the prisoner of the Lord, beseech you to walk worthy of the calling with which you were called, with all lowliness and gentleness, with longsuffering, bearing with one another in love, endeavoring to keep the unity of the Spirit in the bond of peace.
and how to stay together as a united, believing church:
Ephesians 4:11 And He Himself gave some to be apostles, some prophets, some evangelists, and some pastors and teachers, for the equipping of the saints for the work of ministry, for the edifying of the body of Christ, till we all come to the unity of the faith and of the knowledge of the Son of God, to a perfect man, to the measure of the stature of the fullness of Christ; that we should no longer be children, tossed to and fro and carried about with every wind of doctrine, by the trickery of men, in the cunning craftiness of deceitful plotting, but, speaking the truth in love, may grow up in all things into Him who is the head—Christ— from whom the whole body, joined and knit together by what every joint supplies, according to the effective working by which every part does its share, causes growth of the body for the edifying of itself in love.
Most of the rest of this letter is about how to stay together in unity, including believers with each other, husbands with their wives, children with their parents, fathers with their sons, and servants with their masters.