All of this is so that in the Fullness of Time He would:
Reconcile and gather all things together in Christ
“For it pleased the Father that in Him all the fullness should dwell, and by Him to reconcile all things to Himself, by Him, whether things on earth or things in heaven, having made peace through the blood of His cross. And you, who once were alienated and enemies in your mind by wicked works, yet now He has reconciled in the body of His flesh through death, to present you holy, and blameless, and above reproach in His sight—” (Colossians 1:19–22, NKJV)
“having made known to us the mystery of His will, according to His good pleasure which He purposed in Himself, that in the dispensation of the fullness of the times He might gather together in one all things in Christ, both which are in heaven and which are on earth—in Him” (Ephesians 1:9–10, NKJV).
“Now all things are of God, who has reconciled us to Himself through Jesus Christ, and has given us the ministry of reconciliation, that is, that God was in Christ reconciling the world to Himself, not imputing their trespasses to them, and has committed to us the word of reconciliation” (2 Corinthians 5:18–19, NKJV).
The goal and purpose as stated in these passages is that God has purposed in eternity past that everything will be reconciled back to the Son of God. We live in a fallen world, and mankind is in rebellion against God. There’s been a separation. But God is going to bring it all back eventually. Every knee will ultimately bow to Christ. Christ is to be all in all.
Put an end to all rule and authority
“Then comes the end, when He delivers the kingdom to God the Father, when He puts an end to all rule and all authority and power” (1 Corinthians 15:24, NKJV).
All other competing powers will be brought under His control.
Destroy death, the last enemy
“The last enemy that will be destroyed is death” (1 Corinthians 15:26, NKJV).
Show the exceeding riches of His grace and display the manifold wisdom of God.
God’s plan from eternity past is that His Son and the people He has redeemed will be a display for all creation to see. It will display how great and inexhaustible the wisdom of God are and how amazing His grace is that He would take rebellious treasonous subjects and transform them by His love and grace into His sons and daughters, adopted into His family and made co-heirs with His Son Jesus Christ.
“That in the ages to come He might show the exceeding riches of His grace in His kindness toward us in Christ Jesus” (Ephesians 2:7, NKJV).
“To the intent that now the manifold wisdom of God might be made known by the church to the principalities and powers in the heavenly places” (Ephesians 3:10, NKJV).
Jesus Christ will reign forever and ever with His bride, the Church
“And they sang a new song, saying: “You are worthy to take the scroll, And to open its seals; For You were slain, And have redeemed us to God by Your blood Out of every tribe and tongue and people and nation, And have made us kings and priests to our God; And we shall reign on the earth”” (Revelation 5:9–10, NKJV).
“Then the seventh angel sounded: And there were loud voices in heaven, saying, “The kingdoms of this world have become the kingdoms of our Lord and of His Christ, and He shall reign forever and ever!”” (Revelation 11:15, NKJV).
“There shall be no night there: They need no lamp nor light of the sun, for the Lord God gives them light. And they shall reign forever and ever” (Revelation 22:5, NKJV).
God will be all in all and will dwell with His people
“Now when all things are made subject to Him, then the Son Himself will also be subject to Him who put all things under Him, that God may be all in all” (1 Corinthians 15:28, NKJV).
“And I heard a loud voice from heaven saying, “Behold, the tabernacle of God is with men, and He will dwell with them, and they shall be His people. God Himself will be with them and be their God” (Revelation 21:3, NKJV).
The mission will have been accomplished. God will have demonstrated His glory and the centrality and supremacy of His Son by creating and sustaining all things through Him, showing His grace by reconciling sinful people to Himself, by demonstrating His justice by not overlooking sin but punishing it in Christ, and ultimately reconciling everything together to Himself so that God Himself will dwell forever with His people.
Link to video on youtube: https://youtu.be/7979yF2hm6s