Here's a [comment] (https://www.reddit.com/r/AskHistorians/comments/3yencp/how_did_christians_react_to_losing_so_many/cydacgx) I wrote earlier that partly addresses this question.

In a nutshell, there were little if any efforts to convert Christians once they came under Muslim rule in 711-720. There were Christian and Jewish communities in Al-Andalus (Muslim-ruled Spain) and followers of these faiths enjoyed special protection as 'People of the Book'. But they did have to pay a tax called jizya that was conceptualized as being the price for that protection. All in all they had their own communities and their own judges who dispensed justice based on their own laws (which in the case of Christians was the Lex Wisigothorum). Muslim judges only had jurisdiction over them in cases that involved other Muslims.

Over time many Christians would convert out of convenience, to avoid paying the jizya and to better their promotion prospects. Add these converts to Muslim settlers from North Africa, the Middle East and Arabia and to Muslim mercenaries that served in the armies (mostly recruited from imported slaves of Slavic origin) and in a few centuries you get a Muslim majority, particularly in the countryside - although some regions seem to have remained predominantly Christian.

Mos bigger cities would, however, keep an active Christian minority up to the moment of their reconquest by Christian kings of the North. The most notable case is that of Toledo, the former Visigothic capital of Spain (reconquered by Castile in 1085).

After the fall of Toledo local Muslim rulers of Spain invited a radical Islamic power from North Africa, the Almoravids, to help them stem the tide of Christian (re)conquest. Now these guys - and the even more fanatical Almohads that took their place later on - went about persecuting local Christians and Jews so a lot of them fled north into Christian lands. So from the 12th century on we can speak of Muslims being the absolute majority in what was left of Al Andalus.

Hope this helps