Ephesians 2 describes a peace between Jew and Gentile that is real and accomplished, even while Christians keep going to war with one another. The text names that gap between the already and the not yet plainly. Avoiding the war Paul warns against means living out the peace God has already given.
Two central theses anchor 2:19 against the false teaching of 2:18. Christ himself is the foundation of God that ‘standeth firm,’ a person and more than a doctrinal claim. Holding to him keeps the church from being shaken by the world.