ROME—The Vatican is giving the public a rare, if temporary, peek inside its secret archive.

On Wednesday the Vatican kicked off a six-month exhibit at Rome's Capitoline Museum that will place 100 documents, usually locked away in the pope's personal archive, on display.

A papal bull excommunicating Martin Luther from the Catholic Church; the judicial acts of the trial of Galileo Galilei; and a letter from the guillotine-bound Marie Antoinette, are just a few examples of the kind of history-in-the-making documents that rarely are seen by the general public.

The exhibit is part of a Vatican effort to rebrand one of its controversial institutions. The Vatican Secret Archives—the official name of the papal archive—have long been a source and symbol of intrigue, providing endless fodder to conspiracy theorists and a useful backdrop to at least one Dan Brown novel.

Critics have long accused the Vatican of using the archive to keep historically relevant—and potentially damaging—documents sealed away for ages.