Explanation: This speech calls on Christian warriors to defend the Byzantine Empire and retake Jerusalem from Muslim armies. Eliminate (A) because there is nothing explicitly stated about the Mongol invasions in the passage, and the Mongols did not appear in the Middle East until the 1250s. Eliminate (B) because the Protestant Reformation is not centered around Jerusalem and did not take place until the early 1500s. Eliminate (C) because the Age of Discovery was not centered around Jerusalem and Constantinople. The answer is (D).