If you wanted to encapsulate into two words the focus and goal of God’s eternal purpose for humanity—His mystery kept secret since the foundation of the world and revealed through the gospel—“sons” and “heirs” would work.
Sonship: “having predestined us to adoption as sons by Jesus Christ to Himself, according to the good pleasure of His will…in Him we have redemption through His blood, the forgiveness of sins, according to the riches of His grace” (Ephesians 1:5, 7).
Heirs: “In Him also we have obtained an inheritance” (Ephesians 1:11).
Jesus’ role in God’s eternal purpose was two-fold. First, it was to model divine sonship, to leave a clear example of how an obedient son submits to his father’s will in all things. Jesus is the prototype of sonship—unconditionally loving God with all His heart, soul, mind, and strength.
In addition, for a sinner dead in sin to become a living son of God, redemption from sin’s slavery and death was essential. The legal and righteous payment of a ransom had to be made to propitiate (satisfy) God’s holy and just wrath against all unrighteousness and ungodliness. Jesus, the Lamb of God, gave His life as a ransom for many (Mark 10:45). His death and resurrection from the dead declared Him to be the Son of God with power (Romans 1:4); they also assure us of the possibility of spiritual regeneration. We who are dead in our trespasses and sins can be made alive together with Christ as sons of God (Ephesians 2:1, 5). The same divine power which resurrected Jesus from the dead can regenerate the spiritually dead.
If a son of God follows Jesus’ example of obedience, He will receive an inheritance. It is what God predestined (Ephesians 1:11-12) for those who follow the Lamb wherever He goes (Revelation 14:4). It is the eternal salvation He gives to all who obey Him (Hebrews 5:9), faithful to Him unto death (Revelation 2:10).
Because of God’s perfect providence in Christ, those who trust and obey the gospel will become His sons. If they hold fast to that word (1 Corinthians 15:2), they will be given an eternal inheritance when this life is over and will have an eternity to praise God and His glory for His manifold wisdom and grace.