Why could only God put our old Adam to death in the crucifixion with Christ?

Les explains that the part of us dealing with eternity is invisible, and no human can touch the invisible. Only God can deal in that area. The old Adamic nature had to be crucified and put to death, but this is a spiritual realm, an invisible part of us, so only God can do it. Only God could put our old Adam to death in that crucifixion with Christ. Likewise, only God can give us the new creative personality, the new nature, because it too is in the area of the invisible. We are a new work of creation, which God alone can do.