Question: Why was the "is/subsists in" change made at Vatican II and what is the significance of the change?
Answer:
The Dogmatic Constitution on the Church from VCII (LumenGentium) stated that the Church of Christ "subsists in" the Catholic Church. This sounds weaker in English than it does in Latin. The verb "subsistare" in Latin means literally to stand beneath or behind in the way that a foundation stands under a building. What Lumen Gentium was saying in paragraph 8 was that the Catholic Church is grounded upon the true Church of Christ in it's fullness. The document acknowledged further on that other groups of Christians also share in "many elements of sanctification and of truth [which] are found outside of [the Catholic Church's] visible structure." It also recognized in Chapter 15 that all validly Baptized persons have some relationship to the Catholic Church. Consequently, to say that the Catholic Church "is" the one true Church of Christ is not entirely accurate. The Catholic Church has in fullness what other groups have in part. In Chapter 14, the same document makes it clear that being a member of the Catholic Church is necessary for salvation:
14. This sacred Council wishes to turn its attention firstly to the Catholic faithful. Basing itself upon Sacred Scripture and Tradition, it teaches that the Church, now sojourning on earth as an exile, is necessary for salvation. Christ, present to us in his body, which is the Church, is the one Mediator and the unique way of salvation. In explicit terms he himself affirmed the necessity of faith and Baptism (cf. Mk 16:16; Jn 3:5) and thereby affirmed also the necessity of the Church, for through Baptism as through a door men enter the Church. Whosoever, therefore, knowing that the Catholic Church was made necessary by Christ, would refuse to enter or to remain in it, could not be saved. They are fully incorporated in the society of the Church who, possessing the Spirit of Christ, accept her entire system and all the means of salvation given to her, and are united with her as part of her visible bodily structure and through her with Christ, who rules her through the Supreme Pontiff and the bishops. The bonds which bind men to the Church in a visible way are profession of faith, the sacraments, and ecclesiastical government and communion. He is not saved, however, who, though part of the body of the Church, does not persevere in charity. He remains indeed in the bosom of the Church, but, as it were, only in a "bodily" manner and not "in his heart." All the Church's children should remember that their exalted status is to be attributed not to their own merits but to the special grace of Christ. If they fail, moreover, to respond to that grace in thought, word and deed, not only shall they not be saved but they will be the more severely judged. Catechumens who, moved by the Holy Spirit, seek with explicit intention to be incorporated into the Church are by that very intention joined with her. With love and solicitude, mother Church already embraces them as her own. So, the Catholic Church does not have a monopoly on all the members of Christ's Church on earth, but membership in her is the ordinary means of salvation and it is obligatory for Christians to join her once they become aware of this.
Art Sippo
The Catholic Legate