On the Nature of the Church

While still prefect for the Congregation for the Doctrine of the Faith, then-Cardinal Joseph Ratzinger signed off on this letter.  The following bit is quite cool:

Moreover, one's belonging to a particular Church never conflicts with the reality that in the Church no-one is a stranger: each member of the faithful, especially in the celebration of the Eucharist, is in his or her Church, in the Church of Christ, regardless of whether or not he or she belongs, according to canon law, to the diocese, parish or other particular community where the celebration takes place. In this sense, without impinging on the necessary regulations regarding juridical dependence, whoever belongs to one particular Church belongs to all the Churches; since belonging to the Communion, like belonging to the Church, is never simply particular, but by its very nature is always universal.

The rest of the letter is also well worth a read, especially for anyone who wants to know what the Catholic Church's understanding of Christ's Church is (for more, see also Lumen Gentium.)