Brothers and sisters, Christ is our peace, He has made both groups (Jews and Gentiles) into one and has broken down the dividing wall, that is, the hostility between us. In His flesh, He has abolished the Law with its commandments and ordinances, that He might create in Himself one new humanity in place of the two, thus making peace, and might reconcile both groups to God in one body through the Cross, thus putting to death that hostility through it. So He came and proclaimed peace to you who were far off and peace to those who were near, for through Him both of us have access in one Spirit to the Father. So then you are no longer strangers and aliens, but you are citizens with the saints and also members of the household of God, built upon the foundation of the apostles and prophets, with Christ Jesus Himself as the cornerstone. In Him the whole structure is joined together and grows into a holy temple in the Lord, in Whom you also are built together into a dwelling place for God in the Spirit.
The two groups Saint Paul is talking about here are the Jews and the Gentiles. The word Gentile means anyone who is not Jewish. Therefore, from Jewish perspective, the two groups make up the whole world. People used to be (and I would argue still are) very tribalistic. Each tribe concerned itself only with its own members. All those on the outside were just that - outsiders.
There was a dividing wall of hostility between the two groups of Jews and Gentiles. The Jews were God-chosen people. Chosen with a specific intentions, for a mission - to prepare the whole universe for the coming Messiah. Paul tells the Ephesian Christians, who were Gentiles, that formerly they were "without Christ...and without God in the world" (Ephesians 2:12), but now they have been brought near.
What or who brought them near? And near to what?
They were "brought near by the Blood of Christ" (Ephesians 2:13), that is, His saving death on the life-giving Cross. Everyone is brought near to God now, to salvation through the faith in Jesus Christ. Christ said that He came to save the whole world, everyone who is ready to repent - align their life with God's will.
Now, God-chosen people are those who accept the baptism and follow Him. The new humanity, the new creation that flows from sacramental union with Christ, is offered to all, whether Jew or Gentile. That's what Paul meant by saying that Christ broke down the dividing wall of hostility between Jews and Gentiles, or even between any and all tribes.
The two groups have now been made into one, the tribalistic categories that used to separate humanity have been transcended so now there is one new humanity. In Christ everyone is equal. Each one of us is still a unique person, but this uniqueness does not separate us any longer, it unites us in Christ.
All those who are in Christ now, meaning in His Church, have access in one Spirit to the Father. All of this presupposes that Christ is our Lord, that we have repented and taken up our cross, denied ourselves, and follow Him. Christ has done everything Paul describes above, but for that to have effect in our life we need to be part of His Body, or at the very least, be on the way to being there.
Another important aspect of Christ breaking down the dividing wall of hostility is that, if we are truly part of Christ's Body, then there can be no hostility between us. If there is still hostility between us, then maybe we are under a great illusion that we are part of His Body?
Yours in the Lord,