The function of religion is the effect and development that religion has in human society. Religion has two important functions in individual sphere. First, increasing knowledge, Avicenna believes that only prophets can become receptacle of truths without been trained. Second, soothing, Avicenna believes that right beliefs are the causes of health. The most important social function of religion is legislation. Avicenna believes that religion is necessary for legislation and organizing the society. Another social function of religion is building solidarity and unity in society. Avicenna forbids what leads to weakening the biggest base of civilization. From other scholars’ point of view, some functions of religion are exclusive and there is no scientific, philosophical or mystical alternative to them.