"Syllabus of Errors" was written by Pope Pius IX in 1864.  Actually "Mirari Vos" written by Pope Gregory XVI in 1832---condemned liberalism as "this false and absurd maxim, or better this madness, that everyone should have and practice freedom of conscience," 


Citation:
"Acts of Gregory XVI,' p. 168, in Hubert Jedin, ed. HISTORY OF THE CHURCH [New York: Crossroad, 1982], vol. VII, p. 287.