For the Catholic mind, Church is to be understood solely as the community of those who communicate together in the body and in the word of the Lord, and a community of communicants exists, just as does a linguistic community, only if there is unity in the witness of its members.
Conversely it must be said that, given this understanding of Church, there neither can nor should be any disavowal of the presence of Christ and of Christian values among separated Christians. If, on the one hand, it is possible to summarize by saying that the Church is a community of communicants under the authority of the Bishop or Rome, who holds the office of primary witness established by the Lord, and that, as such, she is visible and unique, with clearly definable boundaries, nevertheless, Catholic theology must state more clearly than ever before that, along with the actual presence of the word outside her boundaries, “Church” is also present there in one form or another; that, furthermore, the boundaries of the efficacy of the Holy Spirit are not congruent with those of the visible Church. For, on the one hand, the Spirit, the grace, on whose action the Church depends for her very existence, can be wanting even those within the Church; on the other hand, it can be efficacious in those outside the Church.
To borrow Congar’s cogent phrase, it would be both foolish and perverse to identify the efficacy of the Holy Spirit with the work of the ecclesial apparatus. This means that even in Catholic belief the unity of the Church is still in the process of formation; that it will be totally achieved only in the eschaton, just as grace will not be perfected until its effects are visible – although the community of God has already begun to be visible. Thus the Catholic knows that he is linked in one hope with his separated Christian brethren: in the hope of one Kingdom of God, in which there will be no further division because God will be all in all (1 Cor 15:28)
From: Das neue Volk Gottes, p.42