As the last Adam, Christ is the sum total of humanity; as the second Man, he is the Head of a new race. So we have here two unions, the one relating to his death and the other to his resurrection. In the first place his union with the race as “the last Adam” began historically at Bethlehem and ended at the cross and the tomb. In it he gathered up into himself all that was in Adam and took it to judgment and death. In the second place our union with him as “the sceond man” begins in resurrection and ends in eternity – which is to say, it never ends – for, having in his death done away with the first man in whom God’s purpose was frustrated, he rose again as Head of a new race of men, in whom that purpose shall be fully realized.
When therefore the Lord Jesus was crucified on the cross, he was crucified as the last Adam. All that was in the first Adam was gathered up and done away in him. We are included there. As the last Adam he wiped out the old race; as the second Man he brings in the new race. It is in his resurrection that he stands forth as the second Man, and there too we are included. “For we have become united with him by the likeness of his death, we shall also be by the likeness of his resurrection.” (Romans 6:5). We died in him as the last Adam; we live in him as the second Man. The Cross is thus the mighty act of God which translates us from Adam to Christ.
(from The Normal Christian Life)