Ephesians 1:13: “In Him, you also, after listening to the message of truth, the gospel of your salvation--having also believed, you were sealed in Him with the Holy Spirit of promise.”
Having believed in the Gospel, you were sealed in Christ by the Holy Spirit. So what is the sealing?
1st Corinthians 6:16-17 teaches that just as a man and a woman become “one flesh,” so does the believer become “one spirit” with God upon being sealed into the Body of Christ, with the indwelling of the Holy Spirit as the temple of God. (1st Corinthians 3:16)
1st Corinthians 6:16-17: “Or do you not know and realize that when a man joins himself to a prostitute, he becomes one body with her? The two, it is written, shall become one flesh. But the person who is united to the Lord becomes one spirit with Him.”
Becoming in Christ is the gateway to regeneration, the indwelling, a new heart and a new spirit as the new creature, when the old creature passes away. I argue that Calvinism places the cart before the horse, when they place cart of “in Christ” before the horse of believing in the Gospel, as per Ephesians 1:13. Yet, Calvinism absolutely requires that a person become preemptively “in Christ” so that they can get the “new heart” that is alone reserved in Christ, in order that their decision for repentance be made irresistible as in Calvinist, Irresistible Grace. This is why the timing of when a person becomes in Christ is absolutely pivotal.