How the Dogma of the Immaculate Conception Was Defined
Divinely revealed truths do not change.
But sometimes it takes a while to figure out what those truths are.
It took the Church hundreds of years before it finally settled the question of who exactly Jesus Christ was in the most basic terms of the question—God? Man? One person or two? (The Council of Chalcedon, in 451, hashed out what has been the Church’s final answer.) It also took hundreds of years before the Church affirmed, in a dogmatic definition, the divinity of the Holy Spirit (Council of Constantinople, 381).
Then there is the definition of the dogma of the Immaculate Conception. That took nearly two millennia. Or, 1854 years after the birth of Christ to be exact.