Everyone here must order here and read Card. Manning's sermon on Bl. Pope Pius IX's Syllabus of Errors.

Given a few years after the promulgation of the Syllabus, Card. Manning begins his sermon by quoting John 6:60 on how the transubstantiation is a "hard saying;" so, too, is the Syllabus a "hard saying." Card. Manning in no way says the Syllabus should be interpreted only in the context of the original docuмents from which the propositions come (as Card. Newman, Fr. Cartechini, et al. say). He bluntly says:

Now these eighty errors are partly in matters of faith, partly in matters of morals; in both of which, as you know, the Catholic Church, and the Head of the Catholic Church also, by Divine assistance, are infallible; that is, they are the ultimate interpreters of the faith, and the ultimate expositors of the law of God, and that not by the light of human learning only, but by the light of Divine assistance, which secures from error.

Coming from the First Vatican Council father responsible for its definition on papal infallibility, Card. Manning's statement on the infallible nature of the Syllabus carries much authority. He continues:

Now I may be asked, "Why should the Holy Father touch on any matter of politics at all?" For this plain reason: because politics are a part of morals. […] Politics are morals on the widest scale.

In the rest of the sermon, he defends Bl. Pope Pius IX's condemnation of the Syllabus's last error:

80. The Roman Pontiff can, and ought to, reconcile himself, and come to terms with progress, liberalism and modern civilization.—Allocution "Jamdudum cernimus," March 18, 1861.

I truly have been meaning to read more of Cardinal Manning! Good to hear about!

His classic is: The temporal mission of the Holy Ghost : or, Reason and revelation (1865)

This is an absolute gem of a find! Thank you Geremia.