The Church cannot be understood just by examining her visible elements. An institution, the Church is particularly a living organism, with spiritual and mystical dimensions which are much more important than any human analysis reveals.
"The Church is the Body of Christ. Through the Spirit and his action in the sacraments, above all the Eucharist, Christ, who once was dead and is now risen, establishes the community of believers as his own Body" (CCC 805).
The CCC (no. 837), under the title of "Who belongs to the Catholic Church?", teaches that full incorporation into the Church is possessed by those who "by the bonds constituted by the profession of faith, the sacraments, ecclesiastical government, and communion, are joined in the visible structure of the Church of Christ". This of course means that others, without being fully incorporated, still belong in some way to the Church. All of those in fact who have received Baptism begin to live the life of Jesus Christ and become *de facto members of the People of God (cf. c. 204). But non-Catholic Christians, though participating in the life of Christ, do not enjoy all the benefits or means of sanctification that Jesus wished his followers to have, for the completion of their growth in him (cf. Eph 4:12-13).
When we say in the Creed that we believe the Church to be one, holy, catholic and apostolic, we are affirming fundamental features by which the Church founded by Christ can be recognized.
The Church is one. Oneness refers both to the "uniqueness" and the unity of the Church. The Church founded by Christ is necessarily unique, for he founded just one Church (it follows that the present multiplicity of christian churches is not according to the will of Christ). He also intended that his Church be one - united and undivided (cf. Jn 10:16) - as a reflection of his own unity with his Father. He especially prayed for this: Jn 17:21.
Jesus Christ is the light of the world (Jn 8:12), the Savior of all mankind (Jn 4:42). He spent the three short years of his public life teaching his followers. He was their "Magister", their Teacher; for them he had "the words of eternal life" (Jn 6:68).
For each person, the one really important thing is to meet Jesus, to be enlighted by him, to follow him. Even with failures, our efforts will be fruitful if they are directed toward believing our Lord's Revelation and doing his will. We will be on the right track, even though we often run it badly. But if we mistake his words or his will, we may apparently run well, but off the track.