“Union with Christ in his death and resurrection is the element of union which Paul most extensively expounds…if we are united to Christ, then we are united to him at all points of his activity on our behalf. We share in his death (we were baptized into his death), in his resurrection (we are resurrected with Christ), in his ascension (we have been raised with him), in his heavenly session (we sit with him in heavenly places, so that our life is hidden with Christ in God), and we will share in his promised return (when Christ, who is our life, appears, we also do appear with him in glory) [Rom 6:14; Col. 2:11-12, 3:1-3].
“This, then, is the foundation of sanctification in Reformed theology. It is rooted, not in humanity and their achievement of holiness or sanctification, but in what God has done in Christ, and for us in union with him. Rather than view Christians first and foremost in the microcosmic context of their own progress, the Reformed doctrine first of all sets them in the macrocosm of God’s activity in redemptive history. It is seeing oneself in this context that enables the individual Christian to grow in true holiness.”
(Dr. Sinclair Ferguson, Christian Spirituality: Five Views of Sanctification, Ed. Donald Alexander, Downers Grove, IL: IVP, 1988).