This is the beginning of perhaps the most famous of Leo XIII.'s many encyclicals, entitled "Rerum novarum" from 1891. To my knowledge it is the first of many papal encyclicals on social issues. It thus marks an important point in church history (and beyond that in the history of ideas in general).

For further details see the encyclical's Wikipedia entry at https://de.wikipedia.org/wiki/Rerum_Novarum

For the text itself: https://www.vatican.va/content/leo-xiii/en/encyclicals/docum...