The Middle-East and North Africa was Christian heartland from Roman times. Christian lands hundreds of years before the birth of Muhammad. Conquered by invading Arab and Turkish Muslims from Late Antiquity until the Fall of Constantinople and increasingly purged of Christians continuing to this day. The Crusades were an attempt by the Church to take back what had been Christian territory since Constantine, and had been European-controlled lands since the days of Alexander the Great.
That the Caliphates were centers of tolerance is not corroborated by modern scholarship (see 'The Myth of the Andalusian Paradise: Muslims, Christians, and Jews under Islamic Rule in Medieval Spain'
https://www.amazon.com/Myth-Andalusian-Paradise-Christians-Medieval/dp/1610170954). Jews and Christians were treated harshly by their Islamic rulers in Spain and one of the reasons for the First Crusade was the ill treatment of pilgrims in Jerusalem by the Seljuk Turks. Contrary to how they are often portrayed today the Frankish crusaders treated their non-Christian subjects with a lot of leniency (confirmed by Arab sources such as Ibn Jubayr), allowing complete religious freedom and Muslim villages to govern themselves locally according to Sharia. Even military orders such as the Knights Templar were known for displays of religious tolerance, in one source we have a temple knight showing a Muslim pilgrim the direction to Mecca to help him with his prayer, or by allowing Muslims to pray in the Temple of Solomon.
That the Caliphates were centers of science, culture and learning was not
because of Islam but
despite Islam. The desert-dwelling Arabs (and later the Turks) on the periphery of civilization took over the most developed area in the known world, the splendours of the ancient world, the heritage of the kingdoms of the hellenistic Diadochi, the most advanced provinces of the Roman and Byzantine Empires, themselves inheritors of the ancient Egyptians, Phoenicians, Persians, Hittites, Assyrians, Babylonians, Greeks etc. Rather, it could be viewed as strange that the Islamic world was not more cultured after taking possession of this cultural treasure hoard, perhaps a testament to the backwardness of Arabic religion and culture. Yes, ancient texts (mostly Christian or pagan Greek) were translated and survived to later inspire Christian Europe but it would never have been an issue if the area hadn't been overrun by Muslims in the first place. Many of the most famous Islamic centres of learning were previously Nestorian Christian and pre-Islamic Persian schools and universities (School of Edessa, School of Nisibis, Academy of Gondishapur), or staffed and led by Christian scholars (House of Wisdom in Baghdad) drawing on Byzantine Greek and Persian knowledge.
