This being a somewhat disputed topic today, I found this quotation from John Owen on the relationship of union with Christ and justification and sanctification.
Our union with him is the ground of the actual imputation of his righteousness unto us; for he covers only the members of his own body with his own garments, nor will cast a skirt over any who is not "bone of his bones, and flesh of his flesh." And so he is "of God made unto us righteousness," 1 Cor. i.30. Hence also is our sanctification, and that both as to its principle in a new spiritual nature, and as unto its progress in fruitfulness and holiness. The principle of it is the Spirit itself of life, holiness, and power. From the commentary on Hebrews, volume 4 (The Works of John Owen, volume 20, p. 150).