The Qur'an forbids the worshiping of idols, which means Muslims are not permitted to make images either of Allah or of the prophet. Some Muslims object to any form of representational art because of the inherent danger of idolatry. For this reason, mosques are often decorated with geometric patterns.

A great sin in Islam is something called shirk, or blasphemy. The Qur'an stresses that God does not share his powers with any partner. It warns that those who believe their idols will intercede for them will find that they and their idols will become fuel for hellfire on the Day of Judgment.

Different grades of shirk have been identified in Islamic law. The shirking of custom includes all superstitions, such as the belief in omens. The shirking of knowledge, for instance, is to credit anyone, such as astrologers, with knowledge of the future.

Many of Muhammad's restrictions in the Qur'an were explicit in establishing distinctions between Arabs and Jews as shown, for example, in his dietary rules, which borrowed heavily from the Mosaic Law. The most radical difference between the Qur'an and Mosaic laws has to do with intoxicating beverages. Jews frown on alcoholic beverages, but they do not forbid them entirely; wine is an important element in many Jewish rituals and feasts. However, Muhammad absolutely forbade the use of such beverages.