"Liberalism Is A Sin" quotes St. Thomas Aquinas right out of the blocks. The greatest evil of which man is capable, short of the outright hatred of God Himself ( which is not unheard of among men, especially atheists, communists, etc., although it usually applies only to the damned ) is heresy for it attacks the very root of faith, without which the one, true religion is impossible.
And the worst heresy ( prior to St. Pius X's Pascendi ) is liberalism, which is a precursor of the even more subtle develomental machinations of Modernism.
Taken to its utlimate logical conclusion, liberalism demands nothing of its adovcates to which atheists also may lay claim. Since one relgion is as good as another to the practicing liberal, then all are just as equally right as they are wrong. Hence, nothing in any of them is objectively true since it is all up to the individual to pick and choose his "beliefs" for whatever sentimental, cultural or ideological reasons that may be lying around at the moment.
By way of application, and consistent with Fr. Sarda's final comments, a great number of Nervous Ordeal Catholics, consecrated and lay alike, are deeply imbued with liberalism. Modern "ecuмenism" is heretical on its face and has been condemed for its premises time and again by preconciliar pontiffs. So is "relgious liberty" ( sic! ) and the "new ecclesiology", to say nothing of the "new liturgical movements" and charismatic insanity so popular in Newrome these days.
Kinda puts a 155mm artillery round right into the whole "hermeneutic of continuity / Brave New Church" camp, doesn't it?