All the following taken from the "American Ecclesiastical Review", August 1946.

Question: In the English version of the Encyclical "Mit Brennender Sorge", sent by Pope Pius XI to the German bishops on March 14, 1937, we read: "The believer has an inalienable right to profess his faith and to put it into practice in the manner suited to him." Could not one conclude from this that no matter what religion a person may profess he has a genuine natural right to practice it, without being molested or impeded?

Answer: If Pope Pius XI meant to teach what our questioner concludes from his statement, he certainly departed from traditional Catholic belief and from the clear teaching of his predecessor, Pope Pius IX, who condemned the proposition: "Everyone is free to accept and to profess that religion which, under the guidance of the light of reason, he has judged to be true" (Denzinger 1715). It is incredible that Pope Pius XI intended to teach a doctrine so utterly at variance with Catholic tradition - a doctrine, moreover, which would lead to the strange conclusion that a person has an inalienable right to be wrong.
The only reasonable interpretation of the Pope's words is that he was speaking of the inalienable right of CATHOLICS to profess and to practice their faith in the manner suited to their religious needs. It must be remembered that the pope was denouncing the Nazi government for its restrictions on the Catholic Church, so that it was most natural that he should proclaim the right of the Catholic to practice his religion. It should be remembered, too, that our English word "believer" is not an adequate translation of "Der glaubige mensch," as used in a papal document. This latter phrase is the equivalent of the Latin "fidelis," which in the language of the Church normally means "one who has the Catholic Faith." Similarly, the word "Christian," when used in the Church's official statements, does not signify anyone who accepts Christ as his religious leader, as the word does nowadays in our land. A Christian, in the language of the Church, means a Catholic.