The Lutheran and Reformed conception of union are fundamentally different. Â In the Lutheran scheme, justification happens outside of union. Â For the Reformed, justification is within the scope of union with Christ and is one of the benefits that manifests the believer’s union with Christ. Â That is if you consider the Westminster Standards and John Calvin as faithful expressions of reformed theology.
Q. 69.Â What is the communion in grace which the members of the invisible church have with Christ?
A. The communion in grace which the members of the invisible church have with Christ, is their partaking of the virtue of his mediation, in their justification, adoption, sanctification, and whatever else, in this life, manifests their union with him. (Westminster Larger Catechism, Q. 69)
And then Calvin:
I confess that we are deprived of this utterly incomparable good [justifying grace] until Chirst is made ours. Â Therefore, that joining together of Head and members, that indwelling of Christ in our hearts – in short, that mystical union – are accorded by us the highest degree of importance, so that Christ, having been made ours, makes us sharers with him in the gifts with which he has been endowed. Â We do not, therefore, contemplate him outside ourselves from afar in order that his righteousness may be imputed to us but because we put on Christ and are engrafted into his body – in short, because he deigns to make us one with him. Â For this reason, we glory that we have fellowship of righteousness with him. (Institutes, 3.11.10, emphasis mine).
Now there are differences among the Reformed regarding the precise relationship between the benefits of salvation, but the importance given to union is evident. Â At the most fundamental level, the Scriptural testimony describes the benefits of salvation happening in Christ. Â Being in Christ is none other than being united to him.