In Christ, this little phrase is ubiquitous throughout the writings of St. Paul. If you are a Bible reader this phrase, ‘in Christ,’ will be very familiar to you, and maybe also very encouraging to you, if not somewhat mysterious sounding. Indeed, there is mystery to it (think of John Calvin’s unio mystica), but not so much that we cannot press into it with very fruitful and edifying understanding towards our own spiritual formational understanding of what it means to be children of God.
Karl Barth has a very insightful way of understanding what this phrase means, and it is related, of course!, to his unique doctrine of election; it is also related, more generally, to a doctrine of creation and theological-anthropology. Barth is concerned to highlight the reality that Jesus Christ himself is indeed the ‘first-fruits’ of God’s creation, in his vicarious humanity for us (see Col. 1.15ff); he is concerned to show that Jesus Christ is really what it looks like to be a human being, and not concerned in abstraction, but concerned in the concrete reality of His humanity serving as the ground of human life and the imago Dei who humans were originally created in as images of the image (and now recreated in, in the resurrection of Jesus Christ).
This makes Barth’s conception unique, not because his conception of Christ’s vicarious humanity is outside the bounds of historic Christian orthodox teaching, but unique instead because Barth worked within the Reformed tradition. For the Reformed tradition in general to be ‘in Christ’ relative to soteriological thinking has to do with declarative reality for the elect; it has to do with the elect’s positional relationship to God, as God declares them to be forensically justified in and through the penal substitutionary work of Christ. This is different from Barth’s emphasis. Barth (and TF Torrance et. al.), as I noted above, has more to do with ontological reality; that is, what reality, for Barth, stands as the ultimate ground of what it means to be human? Answering this question, for Barth, is to answer the question: what does it mean to be ‘in Christ?’ Barth’s response is this:
“In Christ” means that in him we are reconciled to God, in him we are elect from eternity, in him we are called, in him we are justified and sanctified, in him our sin is carried to the grave, in his resurrection our death is overcome, with him our life is hid with Christ in God, in him everything that has to be done for us has already been done, has previously been removed and put in its place, in him we are children in the Father’s house, just as he is by nature. All that has to be said about us can be said only by describing and explaining our existence in him; not by describing and explaining it as an existence we might have in and for itself…. For by Christ we will never be anything else than just what we are in Christ. And when the Holy Spirit draws and takes us right into the reality of revelation by doing what we cannot do, by opening our eyes and ears and hearts, he does not tell us anything except that we are in Christ by Christ.
Barth’s concern is bigger than simply being concerned with a doctrine of salvation; he is more focused on the big picture of God’s good creation. Yes, sin entered the picture and humanity’s plight went wild in the wilderness of that sin, but sin was not a subversion of God’s plan nor its dictate. In other words, humanity always already had a ground and location apart from sin, and that ground and reality was, like we noted earlier, the humanity of Jesus Christ, whose image humanity was originally created in and recreated ‘in Christ.’
Barth’s conceiving, then, has more to do with ontology and humanity’s orientation relative to God in that ontology. Enclosed within that reality is where a doctrine of salvation and/or soteriology can be premised and built upon, not the other way around (as the Augustinian method has it, the method upon which Reformed-orthodox theology is built).
 Karl Barth, CD I/2, 240 cited by George Hunsinger, Evangelical Catholic And Reformed: Doctrinal Essays on Barth and Related Themes (Grand Rapids, Michigan/Cambridge, U.K.: William B. Eerdmans Publishing Company, 2015), Loc. 4577, 4583 Kindle.