Eph 1:5 having predestined us to adoption as sons by Jesus Christ to Himself, according to the good pleasure of His will,
The whole line of reasoning here is general instead of particular. God foreordained a church which should be composed of those He adopts as His children. This is according to the pleasure of His will. The act of predestination was actually due to God’s sovereign will. His will was the cause.
The act of adoption illustrates predestination. An adopted child normally doesn’t choose the parent but the parents, according to their free will, chooses the child. “15…you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” 16 The Spirit himself bears witness with our spirit that we are children of God…”(Rom 8:15-17). These few verses among others teach us how we are related to God the Father, it is by adoption. To quote John MacArthur, “… God miraculously gives His own nature to those whom He has elected and who have trusted Christ. He makes them His children in the image of His divine Son, giving them not just Christ’s riches and blessings but also His very nature”.
The new nature gives believers access to limitless blessings by inheritance. Inheritance is reserved for heirs who are sons. God in His rich grace adopts us into His family and freely bestows His inheritance on us. He is also our inheritance and portion. We are heirs and share in all that are His: His love, grace, mercy, wisdom, power, truth, faithfulness etc.