In Paul’s letter to the Ephesians, those promises are truly astonishing. Their origins reach back to the plan formed in God’s mind before time. He planned to select people “in Christ” and to adopt them as His children (Ephesians 1:4-5). The Father gave His own Son’s blood to forgive all who are “in Christ” (Ephesians 1:7). Since they are true children of the Father, He makes them His heirs. He is fully capable of fulfilling this astounding plan.
In [Christ] 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 (Ephesians 1:11).
The past tense, “we have obtained an inheritance,” shows how secure our status is with God. The Holy Spirit seals us, guaranteeing our inheritance, though it is yet future.
The Spirit is the guarantee of our inheritance.
We were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of His glory (Ephesians 1:13-14 cf. Ephesians 4:30).
The Spirit Himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with Him in order that we may also be glorified with Him (Romans 8:16-17).