Aidan Nichols gives a neat summary of the Triune unity of the church in his Figuring out the Church: Her Marks, and Her Masters . Following Heribert Muhlen, he particularly emphasizes the role of the Spirit, who is “one Person in many persons” (27).
More fully: “The Church’s members are not one through hypostatic union with Christ. Rather, the unity of the una quaedam persona comes about through the mediation of the Holy Spirit, who is himself one and the same in Christ and in ourselves. Thus the unity of the Church in Christ is not . . . simply the result of the Father’s predisposing plan. The Father’s predisposing plan is to render a world created and saved a unity in the human species precisely by sending the Holy Spirit to be one single Person in the Word incarnate and ourselves. We - Christ and each other in Church - form una mystica persona , ‘one mystical person’” (29).