This union may be defined as that intimate, vital, and spiritual union between Christ and His people, in virtue of which He is the source of their life and strength, of their blessedness and salvation.Systematic Theology, 449
Another great mystery in the way of sanctification, is the glorious manner of our fellowship with Christ, in receiving a holy frame of heart from Him. It is by our being in Christ, and having Christ Himself in us; not merely by His universal presence as He is God, but by such a close union that we are one spirit and one flesh with Him.https://www.heritagebooks.org/products/the-gospel-mystery-of-sanctification-marshall.html
Obedience, therefore, is not something that may be conceived of artificially or abstractedly. It is obedience that enlisted all the resources of his perfect humanity, obedience that resided in his person, and obedience of which he is ever the perfect embodiment. It is obedience that finds its permanent efficacy and virtue in him. And we become the beneficiaries of it, indeed the partakers of it, by union with him.Redemption Accomplished and Applied, 19
But this "putting to death" of sin is not only demanded by our incorporation into Christ; it is also empowered and effected by it. Union with Christ, because it puts us in a new relationship to sin and brings us into the sphere of the Spirit's power, will impact the way we live. Ultimately, then, the imperative "put to death" in this verse must be viewed as a call to respond to, and cooperate with, the transformative power that is already operative within us.The letters to the Colossians and to Philemon (p. 244). Grand Rapids, MI: William B. Eerdmans Pub. Co.
There is only one genuine cure for legalism. It is the same medicine the gospel prescribes for antinomianism: understanding and tasting union with Jesus Christ himself. This leads to a new love for and obedience to the law of God, which he now mediates to us in the gospel. This alone breaks the bonds of both legalism (the law is no longer divorced from the person of Christ) and antinomianism (we are not divorced from the law, which now comes to us from the hand of Christ and in the empowerment of the Spirit, who writes it in our hearts).The Whole Christ: Legalism, Antinomianism, and Gospel Assurance Why the Marrow Controversy Still Matters (pp. 157). Crossway
Subjectively, the union between Christ and believers is effected by the Holy Spirit in a mysterious and supernatural way, and for that reason is generally designated as the unio mystica or mystical union.Systematic Theology, 447