NCR(eporter) has an informative dialogue between two scholars of the Crusades, Thomas Madden and Carole Hillenbrand:

I want to ask Carole Hillenbrand about something Thomas Madden wrote: “Without the Crusades, it [Christianity] might well have followed Zoroastrianism, another of Islam’s rivals, into extinction.” Carole, do you think that’s true?

Hillenbrand: No. Firstly, Zoroastrianism is not extinct. Zoroastrians, some 130,000 in the world now, are still practicing their faith in Yazd and other parts of southern Iran as well as in Bombay (Mumbai) and the diaspora. When the Arabs conquered the Persian Sasanian empire in the mid-seventh century, that empire totally collapsed. But its state religion, Zoroastrianism, did not die; it was relegated to the status of “tolerated religion.”

Madden: Carole is quite right. Zoroastrianism is not completely extinct. Unfortunately, somewhere in the editing process my word “virtual” dropped out. Nevertheless, Zoroastrianism is no longer a major world religion, which it was on the eve of the Muslim conquests of Persia. That was my point.

Hillenbrand: I would also argue that if the all-conquering Arab armies had failed to eradicate Christianity when they were at their very strongest, it is unlikely that any other subsequent Muslim force in the Middle Ages could have done so either. The coming of the Crusades did not stem further Muslim conquest of Christian territory. The Ottoman Turks in the 15th and 16th centuries took much of the Christian Balkans and famously advanced to the very walls of Vienna. The fear of their advance terrorized much of 16th-century Europe. But they, too, failed to overcome the whole of Christian Europe, even though they captured Constantinople, the greatest of Christian cities, in 1453.

Christianity, in short, proved too hardy a growth to be extirpated by the Muslims.

Madden: The Turks failed to capture Vienna because of a run of freak rainstorms that kept the sultan from bringing his heavy artillery to bear against the city. By any reckoning, the armies of Islam were much more powerful under the Ottoman Turks than the Arabs. The people of 15th- and 16th-century Europe were terrified with good reason — for much of Europe had already been conquered by the Turks. The Crusades were the only organized defense against these relentless attacks. Without centuries of crusading that slowed (but did not stop) the advance of Muslim expansionism, it seems likely to me that Western Europe would have succumbed, just as southeastern Europe did. In that case, Christianity would not have ceased to exist. But it would, in my estimation, have been reduced to a small minority religion — much like Zoroastrianism today.