The pope condemned Modernism, and a whole range of other evolutionary principles concerning Roman Catholic dogma. Pius X instituted commissions to cleanse the clergy of theologians promoting Modernism and some of its (liturgical) consequences.
It enjoined a compulsory Anti-Modernist oath on all Catholic bishops, priests and teachers to force them to come to clear terms with what they believed; this oath remained in force until it was abolished by Pope Paul VI in 1967. Traditionalist Catholics point to this document as evidence that pre-Vatican II popes were highly concerned about enemies of Christendom infiltrating the human element of the Catholic Church.
Pascendi Dominici Gregis English Text