Summary: 1) United in God's Kingdom (Ephesians 2:19a) 2) God’s Family (Ephesians 2:19b) and 3) God's Temple (Ephesians 2:20–22).
Ephesians 2:19-22  So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God  built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, in whom the whole structure, being joined together, grows into a holy temple in the Lord.  In him you also are being built together into a dwelling place for God by the Spirit. (ESV)
Every summer, thousands of people make the dangerous sea journey from North Africa to Europe's Mediterranean coast, often aboard vessels that are poorly equipped for the trip. Many of them attempt the voyage to flee war and poverty in Africa and the Middle East. A group of passengers, consisting of 105 people, left Libya in a rubber boat. Militant Muslims, who were among the migrants trying to flee from Libya to Italy in a boat, threw 12 of their fellow passengers overboard. Witnesses of the incident testified that the perpetrators had committed this crime because the victims were followers of Christ. According to tearful witnesses, a dispute broke out when a Nigerian Christian man prayed for salvation as the boat moved out to sea. Other passengers on the voyage told police that they themselves were spared "because they strongly opposed the drowning attempt and formed a human chain." (https://www.vomcanada.com/News-Articles/)
The unity of believers is not a feel good attempt or mere theological construct. It is a life or death necessity in an increasingly hostile world. Physical threats are backed by evil spiritual forces that try to destroy the unity of the Body of Christ. Satan sows the seeds of discord in an attempt to stir up strife and cause division. Believers that have been brought together by the work of the Holy Spirit can either ignore, frustrate or embrace this unity. The danger is that we frustrate the work that the Spirit has already done by attempting seconding guessing motives or showing distain for others. If we think we can tackle the world on our own, then apart from the pack we are easy pickings from prey. To embrace this unity is to cultivate what the Holy Spirit has already created. He creates life. Our job is to maintain an environment that this may flourish.
In Ephesians 2, Paul closes his discussion of the marvelous unity of the Body of Christ by giving three metaphors to illustrate it. In the picture of 1) Fellow Citizens (Ephesians 2:19a) he shows how Jew and Gentile have become part of the same kingdom. In the picture of 2) God’s household (Ephesians 2:19b) he shows how all believers are one spiritual family in Christ. Finally, in the picture of 3) A Holy Temple in the Lord (Ephesians 2:20–22) he shows that all believers are together a habitation for God.
Believers are a part of the Unity of the Body of Christ being:
1) United in God’s Kingdom (Ephesians 2:19a)
Ephesians 2:19a  So then you are no longer strangers and aliens, but you are fellow citizens with the saints (and members of the household of God) (ESV)
Whether former strangers and outcasts or former aliens and guests, all believers in Christ become fellow citizens of God’s kingdom with the saints—the believers from every age who have trusted in God. Strangers/Foreigners are people outside a country or community, with no special rights or privileges. The word for aliens (paroikos) often is translated as “sojourners,” a term that accentuates the transient nature of the Gentiles. In that condition they were like aliens with an “immigrant visa,” which granted them limited rights and privileges, but not full citizenship or permanent residency (Patzia, A. G. (2011). Ephesians, Colossians, Philemon (p. 200). Grand Rapids, MI: Baker Books.).
God’s kingdom has no strangers or aliens, no second–class citizens. Believers are attached to a heavenly commonwealth (cf. 1:27), that is, a heavenly kingdom. They belong to a heavenly city, the Jerusalem that is above (Gal. 4:26). These Gentile Christians now have a homeland or commonwealth. They ‘belong’ as fellow-citizens with the rest of believers in that heavenly commonwealth ruled by God (O’Brien, P. T. (1999). The letter to the Ephesians (p. 211). Grand Rapids, MI: W.B. Eerdmans Publishing Co.).
Please turn to Philippians 3
Being in the Kingdom of God has responsibilities for fellow citizens. Saints are not higher-level citizens but purposeful co-heirs of redemption with kingdom responsibilities to one another. This is how Paul explains this to the Philippians:
Philippians 3:12-4:1  Not that I have already obtained this or am already perfect, but I press on to make it my own, because Christ Jesus has made me his own.  Brothers, I do not consider that I have made it my own. But one thing I do: forgetting what lies behind and straining forward to what lies ahead, I press on toward the goal for the prize of the upward call of God in Christ Jesus.  Let those of us who are mature think this way, and if in anything you think otherwise, God will reveal that also to you.  Only let us hold true to what we have attained.  Brothers, join in imitating me, and keep your eyes on those who walk according to the example you have in us.  For many, of whom I have often told you and now tell you even with tears, walk as enemies of the cross of Christ.  Their end is destruction, their god is their belly, and they glory in their shame, with minds set on earthly things. But our citizenship is in heaven, and from it we await a Savior, the Lord Jesus Christ, who will transform our lowly body to be like his glorious body, by the power that enables him even to subject all things to himself. [4:1] Therefore, my brothers, whom I love and long for, my joy and crown, stand firm thus in the Lord, my beloved. (ESV)