The Mystery of His Will
The mystery of His will according to his purpose is the sonship by which we receive the Spirit to fulfill Gods plan of returning all things back to their original state:
Ephesians 1:9
making known to us the mystery of his will, according to his purpose, which he set forth in Christ
The missing part of Gods plan, not lost but kept secret by God until the Spirit was given, is that all nations would have access to the Father through the Spirit baptism, thereby making them one and one with God.
Ephesians 1:10 (Wuest An Expanded Translation)
10 with respect to an administration of the completion of the epochs of time to bring back again to their original state all things in the Christ, the things in the heavens and the things on the earth, in Him,
The uniting of the nations in Christ through sonship by the Spirit is the full unveiling of His plan to give humanity dominion over this evil age that righteousness in God’s sight would rule again; and rule this degree-that all things in the heavens and the earth would be returned to their original state.
Ephesians 1:11
In him we have obtained an inheritance, having been predestined according to the purpose of him who works all things according to the counsel of his will,
We have become heirs of God through our inheritance of the complete Spirit of God to dwell in us. This was planned ahead of time and is in accordance with His purpose in verse 10. The counsel of His will does not mean He has a personal will for you. It is the working out of His overarching plan (to return the heavens and the earth back to their original state again) according to His own wisdom and understanding.
Ephesians 1:13
In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit,
This “sealing” in context is still speaking the mystery of His will, to give as an inheritance to every son His promised Spirit (Acts 2 baptism of the Spirit).
Ephesians 1:14
who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory.
The receiving of His Spirit is the down payment until our full adoption as sons when we receive immortal indestructible bodies just like His at Christs return.
Ephesians 1:18
having the eyes of your hearts enlightened, that you may know what is the hope to which he has called you, what are the riches of his glorious inheritance in the saints,
Our transformation as sons from babes to mature men in the image of Christ lies in the grasping and working out of the immeasurable riches of hidden wisdom and revelation hidden in Christ in us through the Spirit.
Ephesians 1:19-20 (ESV)
19 and what is the immeasurable greatness of his power toward us who believe, according to the working of his great might 20 that he worked in Christ when he raised him from the dead and seated him at his right hand in the heavenly places,
As a present result of Christ in you by the Spirit, there is a limitless power source according to our faith that stands ready to be used. This same power in us is the same power that raised Christ from the dead and seated Him as King. This power has also seated you with Christ in dominion over the heavens and the earth.
Ephesians 1:21-23 (ESV)
21 far above all rule and authority and power and dominion, and above every name that is named, not only in this age but also in the one to come. 22 And he put all things under his feet and gave him as head over all things to the church, 23 which is his body, the fullness of him who fills all in all.
The age to come is the kingdom that is “at hand”. This age (in which Christ rules) through His Body in righteous and justice is being established now, and because of that, this present age (where death rules) is already in the process of passing away. He did all these things for His Body that His fullness would fill the earth through it.