17. Therefore, there exists a single Church of Christ, which subsists in the Catholic Church, governed by the Successor of Peter and by the Bishops in communion with him. The Churches which, while not existing in perfect communion with the Catholic Church, remain united to her by means of the closest bonds, that is, by apostolic succession and a valid Eucharist, are true particular Churches. Therefore, the Church of Christ is present and operative also in these Churches, even though they lack full communion with the Catholic Church, since they do not accept the Catholic doctrine of the Primacy, which, according to the will of God, the Bishop of Rome objectively has and exercises over the entire Church.
On the other hand, the ecclesial communities which have not preserved the valid Episcopate and the genuine and integral substance of the Eucharistic mystery, are not Churches in the proper sense; however, those who are baptized in these communities are, by Baptism, incorporated in Christ and thus are in a certain communion, albeit imperfect, with the Church. Baptism in fact tends per se toward the full development of life in Christ, through the integral profession of faith, the Eucharist, and full communion in the Church.
“The Christian faithful are therefore not permitted to imagine that the Church of Christ is nothing more than a collection — divided, yet in some way one — of Churches and ecclesial communities; nor are they free to hold that today the Church of Christ nowhere really exists, and must be considered only as a goal which all Churches and ecclesial communities must strive to reach”. In fact, “the elements of this already-given Church exist, joined together in their fullness in the Catholic Church and, without this fullness, in the other communities”. “Therefore, these separated Churches and communities as such, though we believe they suffer from defects, have by no means been deprived of significance and importance in the mystery of salvation. For the spirit of Christ has not refrained from using them as means of salvation which derive their efficacy from the very fullness of grace and truth entrusted to the Catholic Church”.
Dominus Iesus, (2000).