He formally defined the dogma of the Immaculate Conception and organized the First Vatican Council , which defined the dogma of papal infallibility .
During his pontificate , he convened the First Vatican Council in 1869 , which decreed papal infallibility . The Pope defined the dogma of the Immaculate Conception of the Blessed Virgin Mary , meaning that Mary was conceived without original sin and that she lived a life completely free of sin .