46 First We must lay down a few rules to guide us in the work of reform. Obviously, there can be no question of reforming the essential nature of the Church or its basic and necessary structure. To use the word reform in that context would be to misuse it completely. We cannot brand the holy and beloved Church of God with the mark of infidelity. We must consider our membership in it as one of our greatest blessings. It testifies to our spirit "that we are the children of God." (28) No, it is not pride nor arrogance nor obstinacy nor stupidity nor folly that makes us so sure of being living, genuine members of Christ's Body, the authentic heirs of His Gospel, the lawful successors of the Apostles. It is a firm faith, a joyous conviction. We hold in our possession that great heritage of truth and holiness which characterizes the Catholic Church of the present day, preserving intact the living heritage of the original apostolic tradition. That is our boast, if you like. It is rather our reason for giving thanks continually to God. (29) It is also the reason why we feel ourselves bound by a graver responsibility before God, to whom we are accountable for so great a benefit, and before the Church in which we must arouse this same conviction together with the desire and resolve to guard this treasure, this "deposit," as St. Paul calls it. (30) We have a responsibility too toward our separated brothers and toward all men, so that all may share with us the gift of God.
Source: Ecclesiam Suam (Vatican.va)