The mystical union in the sense in which we are now speaking of it is not the judicial ground, on the basis of which we become partakers of the riches that are in Christ. It is sometimes said that the merits of Christ cannot be imputed to us as long as we are not in Christ, since it is only on the basis of our oneness with Him that such an imputation could be reasonable. But this view fails to distinguish between our legal unity with Christ and our spiritual oneness with Him, and is a falsification of the fundamental element in the doctrine of redemption, namely, of the doctrine of justification.
Louis Berkhof, Systematic Theology, 452. (HT: Brian Chang)