The other is that special conjunction, which is between Christ and Believers; Christ is the Head, they are the Members: the Ligatures of this Mystical Union are the Holy Spirit and Faith, the quickening Spirit (saith the reverend Usher) descends downwards from the head to be in us a fountain of supernatural life; a lively Faith, wrought by the same Spirit, ascends from us upward to lay fast hold upon him. The Scripture notably sets forth this Union, We dwell in Christ and he in us, John 6.56, We abide in him and he in us, John 15.4. We are Members of his Body, of his Flesh, and of his Bones, Ephes. 5.30, 32. And he is in us the hope of Glory, Col. 1.27. This the Apostle calls a great Mystery, and the Riches of the Glory of the Mystery; we are ingrafted into him as Branches into a Root; cemented to him as the building is to the foundation; incorporated with him as the food is with our Bodies; united to him as Members are to the Head. We ead his Flesh, and drink his Blood, and become one Spirit with him; nothing can be more emphatical, the Holy Spirit which resides in him the Head, falls down upon us Members, and so makes a kind of continuity between him and us, too Spiritual and Divine to be interrupted by any local distance: Hence St. Chrysostom saith, that there is no medium or middle between Christ and us; hence St. Austin saith that, Believers are made one Christ with the Man Christ, the Head and the Body make up one Christ: Hence that of Aquinas, that Christ and his Members are but one mystical person; the consequence of this admirable Union is the communication of Divine Blessings from him to us… said the learned Zanchy, All our good things depends on this most necessary Union.
~ Edward Polhill Speculum theologiae in Christo pg. 334-335 (1678)
We should notice that Polhill favorably quotes Aquinas saying that Christ and the Church are one person, but he has also explains that it is the Person of the Holy Spirit that makes this possible. The individual believers don’t become Jesus of Nazareth, nor does Jesus lose his individuality. Rather, the singular person of the Holy Spirit unites them in the one bond of love. As further quotes will show, this is a real union.