The article above claims that Christians and Muslims once got along in Muslim-ruled Spain, and the Crusaders drove a wedge between the two populations.
But even historians who celebrate Muslim Spain admit that most non-Muslims there lived as second-class citizens subject to various forms of legal and social discrimination set down by Islamic law. Non-Muslims had a "protected" or dhimmi status, which meant that, as long as they accepted their inferior social position stipulated by Islamic law, non-Muslims lives and property could not be taken by Muslims.
Somewhat like the Jim Crow laws in the United States, which subjected African-Americans to social stigma and limitation and required humiliating obsequiousness, the "dhimmi" or protected status abased non-Muslims as a population.
And even when non-Muslims accepted that second-class status and "knew their places," their lives and property could still be subject to forfeit if any Muslim claimed that a non-Muslim had violated the dhimma, or protection contract.
Non-Muslim testimony was not equal to Muslim testimony in Islamic courts. Thus, predators could take advantage of non-Muslims, accuse them of blasphemy against Muhammad or the Qur'an, or of other crimes, and the non-Muslims accused would have little legal recourse. Periodically mobs would form and massacre non-Muslims and loot their property.
Sometimes, a Muslim governor would elevate a non-Muslim to a high post in Muslim Spain, but because mainstream interpretations of Islamic law say that a non-Muslim may not rule over a Muslim, the occasional elevation of a gifted non-Muslim to an administrative position in Muslim Spain could provoke massacres by the Muslim population against non-Muslims.
It is an error to say that the Crusades drove a wedge between Muslims and non-Muslims in medieval Muslim Spain. Mainstream interpretations of Islamic law differentiated strongly, and still do differentiate strongly, between non-Muslims and Muslims, and give the latter significantly more rights than the former.