Paul here says several things about the church, and while we are familiar with these concepts it is good for us to remind ourselves of the glory that belongs to the people of God.
First, Paul says that the church is a mystery (vv. 5 and 9). The term ‘mystery’, in its Pauline usage, does not refer to something mysterious. Instead, the term is used to describe truths that have been revealed. At one time, these truths were unknown.
Second, Paul says that the church is a fellowship (v. 6). Paul mentions three features of the church – heirs, members of the body, and partakers of the promise – and adds to each a small prefix, which means ‘together’. Together, God’s people are adopted, they are united together in a living organism (the church), and they share in God’s promise.
Third, the church is a means of instruction for the spiritual authorities (v. 10). These creatures could be either good or bad angels, perhaps both, who inhabit the spiritual realm. In 6:12 they definitely are evil angels, but there is no reason for not including good angels here. They come to church to learn about the wisdom of God, not only in what they hear but also in what they see. In the church they see the triumph of divine grace and the defeat of the powers of evil. They learn about God’s grace through Christians.
Fourth, the church of Jesus Christ gives meaning to human history. God has let us into the secret purpose of reconciliation. In the church, Jews and Gentiles are reconciled; so also are slaves and freemen, males and females, Greeks and Barbarians (Gal. 3:28; Col. 3:11). In the church today God is forming the people who will inhabit the new heavens and the new earth. All other events and movements in history pale into insignificance, apart from the contribution they make towards sinners coming into Christ’s kingdom.
The church is a mystery, a fellowship, a means of instruction to angels, and the key to human history because it is united to Jesus Christ