Modernism refers to theological opinions expressed during the late 19th and early 20th centuries, but with influence reaching into the 21st century, which are characterized by a break with the past. Catholic modernists form an amorphous group. The term "modernist" appears in Pope Pius X's 1907 encyclical Pascendi Dominici gregis. Modernists, and what are now termed "Neo-Modernists," generally, do not openly use this label in describing themselves.