Taken from Rorate Caeli
How can there be degrees in the unity of faith? Either these other religions maintain their faith to be true and will maintain that it alone is true, or they admit the possibility of some other faith, and in that case are not certain of their own, and have not faith.
Whenever a religious system approaches the principle of unity, that is, when it excludes from its bosom all opinions contrary to those which it professes, it is because it is sensible of the absurdity of calling one proposition true, and yet receiving another totally opposed to it.
Every time a religious system departs from the principle of unity, it is because, not being able to find conclusive arguments to establish the certainty of its faith, it grants to others the same tolerance it requires for itself; it dares not exclude others, because they might on the same principle exclude it.
That the Catholic Church is not subject to these fluctuations, but posseses unity of faith in the highest degree, is a proof of the perpetual certainty of faith to be found in her, and of that immutability which is put forth by Catholics as a character of the truth of the faith they profess.
Osservazioni sulla Morale Cattolica