Historians give many reasons for the spread of Islam facilitating trade. Islam established common values and rules upon which trade was conducted. It created a network of believers who trust each other and therefore trade with each other even if they do not personally know each other. Such trade networks existed before Islam but on a much smaller scale. The spread of Islam increased the number of nodes in the network and decreased its vulnerability. The use of Arabic as a common language of trade and the increase of literacy through Qur'anic schools, also facilitated commerce.