Ephesians 2:14 For He is our peace—He who has made Jews and Gentiles one, and in His own human nature has broken down the hostile dividing wall, 15 by setting aside the Law with its commandments, expressed, as they were, in definite decrees. His design was to unite the two sections of humanity in Himself so as to form one new man, 16 thus effecting peace, and to reconcile Jews and Gentiles in one body to God, by means of His cross—slaying by it their mutual enmity. 17 So He came and proclaimed good news of peace to you who were so far away, and peace to those who were near; WEY
Some theorize that this section of Ephesians was part of a hymn. That would involve a bit of speculation since there aren’t any Bibles that offset it as poetry (although it does flow that way). The “dividing wall” (“barrier” or “partition” in other translations) is a reference both to those walls in the Temple that limited access for people and also to the veil splitting that occurred in the Holy of Holies just before Christ died on the cross. This is significant because previously only the High Priest could pass through the veil into the Holy of Holies and did so only once a year. Now all have access to God.
In Jesus’ time, the law had become a way to be saved. So if you were good enough, you could enter into God’s presence with the measurement of your goodness being the law. The problem is that no one can do this perfectly (as Peter points out in Acts 15:10).
In Exodus, the people are afraid of God so they send Moses up the mountain. As time goes by, walls get erected and there is a separation from God. Now there is peace as God wants to reconcile all things in Heaven and Earth.
Ephesians 2:18 For through Him we both have our access in one Spirit to the Father. NHEB
The saving work involves the Father, the Son and the Holy Spirit so while the Bible teaches the Trinity, the actual word itself doesn’t show up in the New Testament. (The hymn stops at this verse according to the theory). Obviously this is really good news for the Gentiles but not as much for the Jews as they have spent centuries persevering in their faith and worship then suddenly these upstarts come along and have the same access to God as they do.
Ephesians 2:19 So then you are no longer strangers and foreigners, but you are fellow citizens with the saints, and of the household of God, 20 being built on the foundation of the apostles and prophets, Christ Jesus himself being the chief cornerstone; NHEB
There is no longer a distinction with some people in the covenant and some outside of it as the God who loves the Jews also loves the Gentiles.
Acts starts off dealing with Jews until Peter goes to the House of Cornelius. As Peter preaches to them about the Gospel, the Holy Spirit falls on the Gentiles to the great astonishment of the Jewish believers.
Acts 10:44 While Peter was still speaking these words, the Holy Spirit fell upon all who heard his message. 45 All the circumcised believers who had accompanied Peter were astounded that the gift of the Holy Spirit had been poured out even on the Gentiles. 46 For they heard them speaking in tongues and exalting God. Then Peter said, 47 “Can anyone withhold the water to baptize these people? They have received the Holy Spirit just as we have!” 48 So he ordered that they be baptized in the name of Jesus Christ. Then they asked him to stay for a few days. BSB
Peter baptizes Cornelius only to be called in by the church in Jerusalem to report about this affair and his reasons for eating with and baptizing the Gentiles. Peter remembers Jesus telling them that they will be baptized with the Holy Spirit. He reasons that if the same thing happens to the Gentiles, then how can he step in and hinder the work of God? Everyone ultimately agrees with Peter and they glorify God for extending this gift of repentance and new life to the Gentiles. (Paul and Barnabas take this to another level as they baptized a great many Gentiles).
In these passages in Ephesians, Paul is leveling the playing field. He isn’t arguing that we should all get along because we are such nice people but rather because we are all sinners saved by the same grace.