Our Foundation (19-20): God made us into one spiritual entity, the church. Even Gentiles who were once excluded by the people of God as "strangers and aliens" are now "fellow citizens."[1] We now all (without distinction) belong to one united family and the "household" of God. We are one indivisible building that shares a single "foundation." This is the bedrock of divine revelation given through the apostles and prophets regarding the person of Jesus Christ.[2] The truth we know and believe about Christ is what binds us together. Our unity and brotherhood depend on a genuine faith in the true Jesus Christ.[3]
Our Purpose (21-22): As a building is comprised of individual parts fitted together, so God's church is made up of individual believers who also relate to one another properly, especially in our common purpose: to grow into a holy temple.[4] And it is as we faithfully abide in Christ ("in whom") that we become a dwelling place for God.[5] As we hear and obey the Son of God ("in the Lord"), and as we live with honesty before the Spirit of God who is in us to sanctify us ("in the Spirit"), we grow[6] into a holy community where God the Father is at home. Even our evangelism fits into this purpose.[7] Sinners believe in Christ and become a part of us, the church, God's holy dwelling place.
[1] The phrase "with the saints" shows this is not simply an inclusion with national Israel, but with Jewish saints in Christ. God grafted regenerate Gentiles into the new covenant meant for regenerate Jews (Jer. 31:31-34).
[2] The two groups are one (one article two nouns) in their testimony regarding Christ. Hence, Paul afterwards identifies Christ as the corner stone. He is whom they testified about (cf. 3:4-5). Cf. also 1 Cor. 3:10-11.
[3] This is not to say that a believer already knows everything about Christ. There is room for growth (4:11-13).
[4] The term "temple [na-os (ναός)]" is specifically the sanctuary/temple proper vs. the temple precincts.
[5] We all stay close to our Shepherd as His listening sheep whether Jew or Gentile (John 10:14-16). As we long for the word of Christ, we are built up as a spiritual house to proclaim His excellencies (1 Pet. 2:1-5, 9-10).
[6] Paul uses the language of progress: "growing into a holy temple" (2:21) and "being built together" (2:22).
[7] Evangelism has this ecclesiological purpose to fold in new saints into the church (4:11-12). Cf. Acts 2:41, 47.