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
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