9 To these rights Pope Leo XIII adds another right regarding the condition of the working class, one which I wish to mention because of its importance: namely, the right to discharge freely one's religious duties. The Pope wished to proclaim this right within the context of the other rights and duties of workers, notwithstanding the general opinion, even in his day, that such questions pertained exclusively to an individual's private life. He affirms the need for Sunday rest so that people may turn their thoughts to heavenly things and to the worship which they owe to Almighty God. No one can take away this human right, which is based on a commandment; in the words of the Pope: "no man may with impunity violate that human dignity which God himself treats with great reverence", and consequently, the State must guarantee to the worker the exercise of this freedom. It would not be mistaken to see in this clear statement a springboard for the principle of the right to religious freedom, which was to become the subject of many solemn International Declarations and Conventions, as well as of the Second Vatican Council's well-known Declaration and of my own repeated teaching. In this regard, one may ask whether existing laws and the practice of industrialized societies effectively ensure in our own day the exercise of this basic right to Sunday rest.
Source: Centesimus Annus (Vatican.va)