"The principle foundation hereof is, that Christ and the Church, in this design, were one mystical person, which state they actually coalesce in, through the uniting efficacy of the Holy Spirit. He is the head, and believers are the members of that one person, as the Apostle declares, 1 Cor. 12:12-13. Hence as what he did is imputed to them, as if done by them, so what they deserved on the account of sin was charged upon him." (p198 of The Doctrine of Justification)
"That our sins were transferred to Christ and made his; that thereon he underwent the punishment that was due to us for them; and that the ground hereof, whereinto its equity is resolved, is the union between him and us, is fully declared in this discourse." (p200)
"This then I say is the foundation of the imputation of the sins of the church to Christ, namely, that he and it are one person, the grounds whereof we must inquire into." (p.201).
p.221: "wherein all the precedent causes of the union between Christ and believers, whence they become one mystical person, centre, and whereby they are rendered a complete foundation of the imputation of their sins to him, and of his righteousness to them, is the communication of his Spirit, the same Spirit that dwelleth in him, to them, to abide in, to animate, and guide the whole mystical body and all its members."
"The Lord Christ's voluntary susception of the state and condition of a surety, or undertaker for the church, to appear before the throne of God's justice for them, to answer whatever was laid to their charge, was required hereto. And this he did absolutely. There was a concurrence of his own will in and to all those divine acts whereby he and the Church were constituted one mystical person. And of his own love and grace did he as our surety stand in our stead before God, when he made inquisition for sin; he took it on himself as to the punishment which it deserved." (p.226)
None of this should be used to suggest or say that we are some how right with God prior to our justification before God. Rather as Christ works, His work is on behalf of His people. He can be placed forth as the covenant head prior to their receiving Him by faith and being justified through faith in Him. On the Cross, God concerns His Son crushing Him for the wickedness of His people, imputing their sins onto Him and propitiating his wrath against them. In His resurrection, because of the union, Christ is "raised up because of our justification." Thus, what happens to the one, has an eye towards those whom God the One represents.