Below is an excerpt from Joseph Ratzinger's work Introduction to Christianity. From pages 342 to 345.
[The Church is Holy]. Is the Church not simply the continuation of God’s deliberate plunge into human wretchedness; is she not simply the continuation of Jesus’ habit of sitting at table with sinners, of his mingling with the misery of sin to the point where he actually seems to sink under its weight? Is there not revealed in the unholy holiness of the Church, as opposed to man’s expectation of purity, God’s true holiness, which is love, love that does not keep its distance in a sort of aristocratic, untouchable purity but mixes with the dirt of the world, in order thus to overcome it? Can, therefore, the holiness of the Church be anything else but the bearing with one another that comes, of course, from the fact that all of us are borne up by Christ?
At bottom there is always hidden pride at work when criticism of the Church adopts that tone of rancorous bitterness which today is already beginning to become a fashionable habit. Unfortunately it is accompanied only too often by a spiritual emptiness in which the specific nature of the Church as a whole is no longer seen, in which she is only regarded as a political instrument whose organization is felt to be pitiable or brutal, as if the real function of the Church did not lie beyond organization, in the comfort of the Word and of the sacraments that she provides in good and bad days alike. Those who really believe do not attribute too much importance to the struggle for the reform of ecclesiastical structures. They live on what the Church always is; and if one wants to know what the Church really is one must go to them. For the Church is most present, not where organizing, reforming, and governing are going on, but in those who simply believe and receive from her the gift of faith that is life to them. Only someone who has experienced how, regardless of changes in her ministers and forms, the Church raises men up, gives them a home and a hope, a home that is hope — the path to eternal life — only someone who has experienced this knows what the Church is, both in days gone by and now.
This does not mean that everything must be left undisturbed and endured as it is. Endurance can also be a highly active process, a struggle to make the Church herself more and more that which supports and endures. After all, the Church does not live otherwise than in us; she lives from the struggle of the unholy to attain holiness, just as of course this struggle lives from the gift of God, without which it could not exist. But this effort only becomes fruitful and constructive if it is inspired by the spirit of forbearance, by real love. And here we have arrived at the criterion by which that critical struggle for better holiness must always be judged, a criterion that is not only not in contradiction with forbearance but is demanded by it. This criterion is constructiveness. A bitterness that only destroys stands self-condemned. A slammed door can, it is true, become a sign that shakes up those inside. But the idea that one can do more constructive work in isolation than in fellowship with others is just as much of an illusion as the notion of a Church of "holy people” instead of a "holy Church" that is holy because the Lord bestows holiness on her as a quite unmerited gift. This brings us to the other word applied to the Church by the Creed: it calls her "catholic". The shades of meaning acquired by this word during the course of time are numerous, but one main idea can be shown to be decisive from the start. This word refers in a double way to the unity of the Church. It refers, first, to local unity — only the community united with the bishop is the "Catholic Church", not the sectional groups that have broken away from her, for whatever reasons. Second, the term describes the unity formed by the combination of the many local Churches, which are not entitled to encapsulate themselves in isolation; they can only remain the Church by being open to one another, by forming one Church in their common testimony to the Word and in the communion of the eucharistic table, which is open to everyone everywhere. In the old commentaries on the Creed, the "Catholic" Church is contrasted with those "Churches" that only exist "from time to time in their provinces" and thereby contradict the true nature of the Church. Thus the word "catholic" expresses the episcopal structure of the Church and the necessity for the unity of all the bishops with one another.