The last Adam.1 Corinthians 15:45
Jesus is the representative head of His people. In Adam every heir of flesh and blood has a personal interest, because he is the covenant head and representative of the race when considered under the law of works; so under the law of grace, every redeemed soul is one with the Lord from heaven, since He is the Second Adam, the Sponsor and Substitute of the elect in the new covenant of love.
The apostle Paul declares that Levi was in the loins of Abraham when Melchizedek met him: It is a certain truth that the believer was in the loins of Jesus Christ, the Mediator, when in eternity the covenant settlements of grace were decreed, ratified, and made sure forever.
Whatever Christ has done, He has accomplished for the whole body of His Church. We were crucified in Him and buried with Him (read Col. 2:10-13), and to make it still more wonderful, we are risen with Him and even ascended with Him to the seats on high (Eph. 2:6). It is in this way that the Church has fulfilled the law and is "blessed in the Beloved."1
She is regarded with satisfaction by the just Jehovah, for He views her in Jesus, and does not look upon her as separate from her covenant head. As the Anointed Redeemer of Israel, Christ Jesus has nothing distinct from His Church, but all that He has He holds for her. Adam's righteousness was ours so long as he maintained it, and his sin was ours the moment that he committed it; and in the same way, all that the Second Adam is or does is ours as well as His because He is our representative.
Here is the foundation of the covenant of grace. This gracious system of representation and substitution, which moved Justin Martyr to cry out, "O blessed change, O sweet permutation!" is the very groundwork of the Gospel of our salvation and is to be received with strong faith and rapturous joy.
1) Ephesians 1:6