"Union with Christ is not to be understood as a “moment” in the application of salvation to believers. Rather, it is a way of speaking about the way in which believers share in Christ in eternity (by election), in past history (by redemption), in the present (by effectual calling, justification, and sanctification), and in the future (by glorification). Nevertheless, our subjective inclusion in Christ occurs when the Spirit calls us effectually to Christ and gives us the faith to cling to him for all of his riches...
"The motif of mystical union has often been presented as an alternative to the forensic (legal) motifs of redemption, especially vicarious substitution and justification. Since Albert Schweitzer, the thesis has repeatedly been advanced, refuted, and then advanced again that justification is a “subsidiary crater” in Paul, while the real central dogma is mystical union. Reginald Fuller notes, “Attempts have been made to pinpoint some other center or focus for Pauline theology, such as ‘being in Christ’ (Schweitzer) or salvation history (Johannes Munck).” However, “Romans, the most systematic exposition of Paul’s thought, clearly makes justification the center.” Not only in Paul but in the pre-Pauline creedal hymns we find this affirmation (2Ti 1: 9 and Tit 3: 4– 5)...
"Like Schweitzer, a variety of contemporary trends in Pauline studies as well as Reformation scholarship are driven by the presupposition that mystical participation in Christ stands over against a forensic emphasis on Christ’s alien righteousness imputed to believers. 3 Through the interpretive lens of union with Christ we can move beyond the false choice of a legal, judicial, and passive salvation on one hand and a relational, mystical, and transformative participation in Christ on the other. Nevertheless, as I argued in relation to Christ’s atoning work, the integral unity of these motifs is possible only because the latter is grounded in the former. As Geerhardus Vos expressed it,
In our opinion Paul consciously and consistently subordinated the mystical aspect of the relation to Christ to the forensic one. Paul’s mind was to such an extent forensically oriented that he regarded the entire complex of subjective spiritual changes that take place in the believer and of subjective spiritual blessings enjoyed by the believer as the direct outcome of the forensic work of Christ applied in justification. The mystical is based on the forensic, not the forensic on the mystical.
"In Romans 5, the covenantal union of humanity in Adam is contrasted with Christ’s covenantal headship, and then in chapter 6 we encounter his most explicit description of union with Christ in his death and resurrection (Ro 6: 1– 23; cf. 1: 3– 4; 4: 25; 1Co 15: 35– 58). Though they have been “in Christ” in God’s electing grace from all eternity (Eph 1: 4, 11; 2Ti 1: 9), their actual union with Christ occurs in time through the work of the Spirit. Throughout the Pauline corpus we encounter this emphasis on union with Christ."[bold emphasis added]
Horton, Michael S. (2011-01-04). The Christian Faith: A Systematic Theology for Pilgrims on the Way (Kindle Locations 14544-14565). Zondervan. Kindle Edition.