7th, It is a native consequence of union with the second Adam, of justification, reconciliation, and adoption. — It is a fruit of vital union with Christ. By vital union with him, we become members of his mystical body, of that body to which he is united, as the glorious Head of sanctifying influences; but as the head is holy, the members must be holy also. Besides, they who are united to Christ are in him; but all who are in Christ are sanctified. "To them that are sanctified in Christ Jesus," 1 Cor. i. 2. In virtue of this union, the believer lives in Christ, and Christ in him: he partakes of the same Spirit that dwells in Christ.
It is a necessary fruit of justification, and inseparably connected with it. It is connected with it in the decree and promise of God, in the offices of Christ, and the design of his obedience unto death. When the blood of Christ is sprinkled on our conscience for justification, it has a special efficacy for sanctification. It purges the conscience from dead works, and then the believing sinner is enabled to serve the living God. "The blood of Jesus Christ his Son cleanseth us from all sin." The sprinkling, or application of the blood of Jesus, has a sanctifying efficacy; for it removes the curse of the law which is the strength of sin, and which hinders the acceptance, both of the sinner's person and performances. When this is done, the dominion of sin is taken away: its power and pollution begin to be gradually removed. Hence are these reviving expressions; "Sin shall not have dominion over you: for ye are not under the law, but under grace." Christ hath redeemed us from the curse of the law, — that we might receive the adoption of sons, and that we might receive the promise of the Spirit through faith.
It is also a fruit of reconciliation. No sooner does the sinner begin to have peace with God in Christ, as one reconciled to him, than a way of free communication is opened between Christ the head of influences, and his soul. Besides, the sanctifying efficacy of the blood of Christ arises from its atoning or pacifying virtue, Heb. ix. 14.
It necessarily follows adoption. "Because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father." "Whom he did foreknow, he also did predestinate to be conformed to the image of his Son; that he might be the first-born among many brethren," Rom. viii. 29.John Colquhoun. Sermons, chiefly on doctrinal subjects, 171-72
In other words...
Union with Christ has the double-benefit of justification and sanctification. But the hallmark of an early modern Reformed doctrine of union with Christ is according theological priority to justification over sanctification, or priority of the forensic over the renovative. Another way to say this is that justification is the legal basis of a believer's redemption. Or still yet, a person can say, "I am sanctified because I am justified." But he cannot say, "I am justified because I am sanctified." - John Fesko. Beyond Calvin: Union with Christ and Justification in Early Modern Reformed Theology (1517-1700), pp 29-30