Continued:
Blessed Pope Pius IX condemned the proposition,
“Every man is free to embrace and profess that religion which, guided by the light of reason, he shall consider true. — Allocution “Maxima quidem,” June 9, 1862; Damnatio “Multiplices inter,” June 10, 1851.”
Notice what is referenced. Perhaps looking at the references will help us understand what is said.
Allocution Maxima quidem says,
"In addition, they dare to deny any activity of God in men and in the world. And they rashly assert that human reason, without any reference to God, is the only judge of truth and falsehood, good and evil, and that human reason is a law unto itself, and suffices by its own natural power for the care of the good of persons and peoples. But since they perversely dare to derive all truths of religion from the inborn force of human reason, they assign to man a certain basic right, from which he can think and speak about religion as he likes, and give such honor and worship to God as he finds more agreeable to himself.
But they indeed arrive at the impiety and effrontery to try to attack heaven and remove God Himself from our midst. With singular lack of principle, equal only to their folly, they do not scruple to assert that there is no all wise and provident Divine Being distinct from the things of this world, and that God is identical to nature, and that He is therefore subject to change; and that God is really coming to be in man and in the world; and that all things are really God and of God’s substance; and that God and the world are really one and the same thing, and so too spirit and matter, necessity and freedom, truth and falsehood, good and evil, just and unjust are all really the same."
It goes on, I encourage you to read more to help understand the context.
Maxima Quidem – The Josias I dunno too much about the various aspects of the website, but the link to the translation (provided it is accurate) should serve to get an idea of context.
Here we can see that reason is used to mean a sort of inherent reason alone, without any reference to God, and that they whom he is writing against assert a “certain basic right…” By which “he can think and speak about religion as he likes, and give such honor and worship to God as he finds more agreeable to himself.”