In the passage just quoted, St. Robert Bellarmine sets out to explain and to define the thesis he is going to defend and explain throughout the rest of the book De ecclesia militante. The outstanding talent of this great Doctor of the Church is precisely his power of forceful and clear exposition. In the section we have just cited, that talent was exercised as perfectly as it is in any section of his works.
St. Robert contends that the one and only supernatural kingdom of God on earth, the ecclesia spoken of in the Scriptures, has been constituted by God as a society composed of members or parts whose appurtenance [going with and belonging - J.G.] to this company is manifest to all men. He asserts that the factors by which a man is constituted as a member or a part of this company are the profession of the true Christian faith, access to the sacraments, and subjection to the Roman Pontiff. The group which is God's one and only ecclesia in this world is actually the company of men who have these factors of unity.
He acknowledges the presence within the Church of faith, hope, charity, and the other supernatural virtues. Furthermore he realizes that these infused virtues themselves constitute another bond of unity with Our Lord and among His disciples. Nevertheless he insists that this spiritual or inward bond of unity is not the factor which constitutes a man as a part or a member of the Church militant of the New Testament.
Yet despite the perfection of St. Robert's teaching and the clarity of his exposition, this section of the second chapter of his De ecclesia militante was destined to be the source of serious and highly unfortunate misunderstanding by subsequent theologians. The weak part of this, perhaps the most important single passage in the writings of any post-Tridentine theologian, was St. Robert's use of the terms "soul" and "body" with reference to the Church.