Blessed John Duns Scotus, the 13th century Franciscan theologian, who espoused the doctrine of Mary's "Preventive Redemption," also known as the Immaculate Conception. This had long been a belief at the popular level, but it was not accepted by theologians until Scotus figured out how to explain it. The Pope says this should be an example to theologians to "always listen to the source of popular faith and maintain the humility and simplicity of children."
Such humility is very important. I am often surprised at some at some of the things that some theologians have the nerve to say about the faith. They seem to think so highly of their own intelligence that they are willing to not only deny, but even ridicule orthodox Christian beliefs. The fact that it is common belief even by those in their own church means nothing. Years ago I saw a news report about some large theology conference, and one of the attendees was interviewed who said that the beliefs of the "man in the pew" is just "folk religion" and has nothing to do with what these real theologians work on. Although I admit that sometimes we can get more technical than some believers would be interested in, theology should never be detached from common faith. Pope Benedict XVI is one of the most brilliant theologians alive today, but he is also a holy and humble man who knows that brilliance is not a guarantee of truth, but can even lead away from the truth if we're not very careful.