It is most concisely stated in the first three chapters of Ephesians. Paul speaks of —
This letter is from Paul, chosen by the will of God to be an apostle of Christ Jesus. I am writing to God’s holy people in Ephesus, who are faithful followers of Christ Jesus. Ephesians 1:1
God decided in advance to adopt us into his own family by bringing us to himself through Jesus Christ. This is what he wanted to do, and it gave him great pleasure. -Ephesians 1:5
God has now revealed to us his mysterious plan regarding Christ, a plan to fulfill his own good pleasure. Ephesians 1:9
Paul repeatedly says God chose, appointed and destined us according to His will.
Note especially Ephesians 1:10 —
And this is the plan: At the right time he will bring everything together under the authority of Christ—everything in heaven and on earth.
The word sometimes translated “plan” is oikonomia, which comes from the word for “house” or “household.” The idea is that of a great household of which God is the Master and which has a certain system of management wisely ordered by Him. Paul here sees God’s plan as a cosmic strategy having to do with all creation.
Five times in Ephesians Paul speaks of “the heavenly realms —
One God and Father of all, who is over all, in all, and living through all. And the same one who descended is the one who ascended higher than all the heavens, so that he might fill the entire universe with himself. – Ephesians 4:6,10