What was the main factor contributing to the growth of the Muslim population in Spain after the time of Muhammad?

The growth of the Muslim population in Spain after the time of Muhammad primarily resulted from the Arab and Moorish military conquests. In the early 8th century, Muslim forces invaded the Iberian Peninsula and rapidly established control over much of the region. This military expansion was not only about conquest but also involved the establishment of Muslim rule, which facilitated the spread of Islamic culture, governance, and religion.

As the Moors settled in Spain, they brought with them their customs, language, and faith. The subsequent establishment of al-Andalus, the Muslim-ruled territory in Spain, fostered a multicultural environment where Islamic practices could flourish. Moreover, the political and social structures set up during this time encouraged the local population to convert to Islam, often as a means to gain advantages in a society led by Muslim leaders.

The other factors, while they might have had some influence, did not play the primary role in increasing the Muslim population. Trade routes existed before and during the Muslim rule, as well as traditional agricultural practices, but they did not directly cause a significant increase in the Muslim demographic. Likewise, though there were some missionary efforts, the decisive factor was the military conquest that initiated and facilitated the proliferation of Islam in the region.