In Pierced for Our Transgressions: Rediscovering the Glory of Penal Substitution, the authors have this to say about the doctrine of union with Christ and it’s critical importance to the doctrine of penal substitution:
Union with Christ is important for the doctrine of penal substitution, for it is on the basis of this union that our guilt is justly imputed to him, and that we are credited with his righteousness and receive all the benefits of his perfect life, sacrificial death and glorious resurrection. As Calvin puts it:
First, we must understand that as long as Christ remains outside of us, and we are separated from him, all that he has suffered and done for the salvation of the human race remains useless and of no value to us. Therefore, to share with us what he has received from the Father, he had to become our and to dwell within us … the holy Spirit is the bond by which Christ effectually unites us to himself.This means, of course, that when Christ received the punishment for our sins, he did not do so as an unrelated third party. He was justly punished for sins that became his on account of his union with sinners. Similarly, when his righteousness was imputed to us, it required no ‘legal fiction’, for his righteousness truly became ours. This answers the objection of some critics who complain that penal substitution is unjust.
As I see it—and I’ve writen this in the margin of the page this is quoted from—union with Christ not only makes penal substitution coherent, it also makes it an argument for definite atonement. If Christ “was justly punished for sins that became his on account of his union with sinners”, then atonement it specific to those who are united with him.
There also an interesting footnote for this quote:
[W]e would distinguish between the union between Christ and believers conceived in God’s mind in eternity and the union we enter into by the Spirit when we put our faith in Christ. Plainly, it is the former union that grounds the imputation of our sin to Christ at the point of the accomplishment of our redemption at Calvary, for only this was then in existence. However, our faith union by which the benefits of Christ are applied to us is not something wholly different, but an outworking of this former union.
Now that I’ve read that, the distinction of the two different kinds union with Christ—the union concieved in God’s mind in eternity in contrast to the union worked by the Spirit at the time of faith—seems obvious, but I’d never thought of it before.
Sometimes the footnotes in books are the best part.