Etsi ecclesia Christi is a papal bull issued by Pope Nicholas V on 30 September 1453 in response to the fall of Constantinople to the Ottoman Empire.Norman Housley, Crusading and the Ottoman Threat, 1453–1505 (Oxford: Oxford University Press, 2012), p. 18.
Nicholas declared that the capture of Constantinople by the Ottomans was the latest in a long line of attacks on Christendom from Islam, whose followers had already conquered the East, Egypt and North Africa. He further claimed that these were the fulfilment of John's prophecies in the Book of Revelation and he called the Ottoman Sultan Mehmed II the "second Mohammed", and a forerunner of the Anti-Christ.Ludwig Pastor, The History of the Popes from the Close of the Middle Ages, Volume II (London: John Hodges, 1891), p. 275. Nicholas called on Christian rulers to defend Christendom with their lives and their money and he granted indulgences to those who, for six months, took part in this crusade.Pastor, p. 276.
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