I've often heard it alleged that 'the Immaculate Conception wasn't a dogma until Pius IX defined it in 1854.' However, I can't reconcile this with what the dogmatic Council of Trent already declared 300 years prior.
If any one does not confess that the first man, Adam [lost original righeousness] ... ... let him be anathema.
If any one asserts, that the prevarication of Adam injured himself alone, and not his posterity ... ... let him be anathema.
If any one asserts, that this sin of Adam,—which in its origin is one, and being transfused into all by propogation, not by imitation, is in each one as his own,— is taken away either by the powers of human nature, or by any other remedy than the merit of the one mediator, our Lord Jesus Christ ... ... let him be anathema.
If any one denies, that infants, newly born from their mothers' wombs, even though they be sprung from baptized parents, are to be baptized; or says that they are baptized indeed for the remission of sins, but that they derive nothing of original sin from Adam ... ... let him be anathema.
If any one denies, that, by the grace of our Lord Jesus Christ, which is conferred in baptism, the guilt of original sin is remitted ... ... let him be anathema.
This same holy Synod doth nevertheless declare, that it is not its intention to include in this decree, where original sin is treated of, the blessed and immaculate Virgin Mary, the mother of God; but that the constitutions of Pope Sixtus IV., of happy memory, are to be observed, under the pains contained in the said constitutions, which it renews.
—Council of Trent, Session V, First Decree, On Original Sin.
CANON 23 — If any one saith, that a man once justified can sin no more, nor lose grace, and that therefore he that falls and sins was never truly justified; or, on the other hand, that he is able, during his whole life, to avoid all sins, even those that are venial, —except by a special privilege from God, as the Church holds in regard to the Blessed Virgin; let him be anathema.
—Council of Trent, Session VI, On Justification.
If this was already dogma per Trent's decree, why did Pope Pius IX define it with an extra-ordinary definition?