I like this translation of this verse, specifically the part "Christ has made me his own." Paul has not obtained the fullness of righteousness or life in Christ, but he presses on toward the upward call of God in Christ Jesus. How does he press on? because Christ has made him his own. The word καταλαμβάνω (katalambanō) means to lay hold of, grasp; to seize, to take possession of; to apprehend to make one own. Let us look at the meaning of such a statement, Christ has made us his own.
he predestined us for adoption as his own... (Ephesians 1:5)
He predestined as his own. He chose us before he created the world, in love (Ephesians 1:4). Just like Jacob, "though they were not yet born and had done nothing either good or bad—in order that God’s purpose of election might continue, not because of works but because of him who calls" (Romans 9:11, ESV). Election is according to grace and it's basis is predestination which is through adoption, where we are made his own, his children. This is through new birth of the Spirit, a Spirit of adoption, which speaks volumes to how he has made us his own and to how we are sealed eternally as his own through predestination.
... also believing we were sealed of the promised Holy Spirit who exists the guarantor of our inheritance to the extent, redemption of his possession... (Ephesians 1:14)
God seals his own as a guarantee until he takes possession of his own. "In Christ we have obtained an inheritance, predestined according to the counsel of his will" (Ephesians 1:11). The Spirit groans inwardly in us as we eagerly await the fullness of our adoption, the redemption of our body (Romans 8:23). It is a profound statement, God chose us, predestined us for adoption as his own, sealing us with the Holy Spirit as a guarantee.
and those he predestined, these also called, and whom he calls, these also justified. Now, whom he justifies these also glorifies (Romans 8:30)
We are eternally secured through predestination.* If he predestined us, it is through adoption in which the Holy Spirit testifies that we are his children, his own (Romans 8:16). Our inheritance is eternally secured and is based on new birth of the Spirit, as Paul writes, "if children then heirs, heirs of God and fellow heirs with Christ, since indeed we suffer with him so that also glorified with him" (Romans 8:17). It's a done deal, it is sealed, if Christ makes you his own, you are a child destined to become an heir, not a servant but a son who abides in the house forever.
because all the ones born of God conquers the world and this is the victory that conquers the world; that our faith. Who is it that conquers the world if not the one believing that Jesus exists the son of God? (1 John 5:4-5)
Who are those who overcome? those born of God. If Christ has made you his own you were predestined to be his own and to overcome. This implies if you are sealed with the Spirit it is eternal. "I will lose none that the Father gives me" (John 6:39). The testimony of the Spirit is also that we have eternal life and this life is in his Son (1 John 5:11-13). Eternal life is more that just going to heaven. If God chose us in love what can separate us from his love? (Romans 8:38-39).
All that the Father gives me will come to me, and whoever comes to me I will never cast out (John 6:37).
We have received eternal life, passing from life to death (John 5:24). And "there is therefore now no condemnation for those who are in Christ Jesus" (Romans 8:1). We were made alive and raised and seated with Christ in the heavenlies (Ephesians 2:5-6). "And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption" (Ephesians 4:30 ESV). It doesn't say if you do you will be unsealed. We are saved by grace and his grace is sufficient to keep us as his power matures us.
now I press own if also to make it my own to the extent like as also Christ Jesus has made me his own (Philippians 3:12)
There are so many other verses we can look at.* But none quite seals it as this verse, Christ has made us his own. It settles the matter. We press on as the chosen sons of God we are, Christ has made us his own. What motivation, what hope, what happiness, what life lies ahead!