Karl Adam wrote :
The Catholic Church as the Body of Christ, as the realization in the world of the Kingdom of God, is the Church of Humanity. Of her essential nature she aims at the incorporation of the men of all times and all places in the one Body of Christ. Hence inevitably her external and internal catholicity, her accessibility and comprehensiveness. And hence also her exclusiveness, that is her claim to be the Church of Humanity, the exclusive institution wherein all men shall attain salvation. Because the Church is conscious that she is the Church of humanity and that Kingdom of God to which all men whatsoever by the will of Christ fundamentally belong, she cannot admit that men can be saved by membership in other societies established by the side of and in antagonism to the primary Church of Humanity founded by Christ. Even Heiler cannot deny the cogency of this position. "So far as Catholicism is genuinely universal and represents fully all religious values, it must be exclusive. But this exclusiveness is not the exclusiveness of narrowness, but of inexhaustible wealth.[1] The Church would belie her own deepest essence and her most outstanding quality, namely her inexhaustible fullness and that which guarantees and supports this fullness, her vocation to be the Body of Christ, if she were ever to recognize some collateral and antagonistic Christian church as her sister and as possessing equal rights with herself. She can recognize the historical importance of such churches. She can designate them as Christian communions, yes, even as Christian churches, but never as the Church of Christ. One God, one Christ, one Baptism, one Church. There can never be a second Christ, and in the same way there cannot be a second Body of Christ, a second manifestation of His spirit. When some American Christians went to Rome in the Spring of 1919 to invite Pope Benedict XV to take part in a “World Conference on Faith and Order,” they misunderstood the Catholic conception of the Church and this its fundamental claim. The Catholic Church can and will appraise generously, and will countenance, all the communities of non-Catholic Christendom. She can and will recognize in them the first rudiments of a preparation for that re-union of all Christians which is demanded by the present state of Christendom in general and of the West in particular. But she cannot recognize other Christian communions as churches of like order and rights with herself. To do so would be infidelity to her own nature, and would be the worst disloyalty to herself. In her own eyes the Catholic Church is nothing at all if she be not the Church, the Body of Christ, the Kingdom of God. This exclusiveness is rooted in the exclusiveness of Christ, in His claim to be the bringer of the new life, to be the way, the truth and the life. The fullness of the Divinity was revealed to us in Christ. The Incarnate God is the last and most perfect self- revelation of God. God’s wisdom, goodness and mercy became incarnate in Him. “Of His fullness we all have received, and grace for grace” (Jn. i, 16). And therefore there is no other road to God except through Christ. There is “no other name under heaven given to men, whereby they must be saved” (Acts iv, 12). But we can grasp Christ only through His Church. It is true that He might, had He so willed, have imparted Himself and His grace to all men directly, in personal experience. But the question is not what might have been, but what Christ in fact willed to do. And in fact He willed to give Himself to men through men, that is by the way of a community life and not by the way of isolation and individualism. He willed that His grace should come to men who were conjoined in a single compact fellowship, and that it should come to them through this fellowship, not without it, and still less in opposition to it. It was not His will to sanctify a countless multitude of solitary souls, but a corporate kingdom of saints, a Kingdom of God. And this method of communicating Himself corresponds entirely to His fundamental requirement, the commandment of fraternal love. For that commandment implies a community, implies the union of the brethren, and there can be no fraternal love without such a community. And it corresponds also to the essential nature of divine grace, which is offered to all men at once. The grace of God in its manifestation is a catholic power, comprehending and grasping all men. So that it cannot manifest itself otherwise than in absolute unity. There can be no contradiction, or dissension, or schism where God is. His truth cannot be otherwise than one truth, one life, one love. And therefore it can be realized in but one form, in a comprehensive fellowship that binds together all men in intimate unity.