Of all Muslim institutions, the mosque is the most important place for the public expression of Islamic religiosity and communal identity. A mosque is a physical manifestation of the public presence of Muslims and serves as a point of convergence for Islamic social and intellectual activity. The Arabic word for mosque is masjid, which means a "place of prostration" before God. Mosques are mentioned in the Qur'an, and the earliest model for a mosque was the residence that the prophet Muhammad built when he moved to Medina. This first mosque was an enclosure marked as a special place of worship. A small part of the mosque was sectioned off to house the Prophet and his family, and the remaining space was left open as a place for Muslims to pray.

Although later mosques developed into complex architectural structures built in diverse styles, the one requirement of all mosques continues to be based on the earliest model: a designation of space for the purpose of prayer. The early mosque served an equally important function that thousands of mosques continue to serve today: The mosque is a place where Muslims foster a collective identity through prayer and attend to their common concerns. A Muslim city typically has numerous mosques but only a few congregational or Friday mosques where the obligatory Friday noon prayers are performed.

As Islam spread outside Arabia, Islamic architecture was influenced by the various architectural styles of the conquered lands, and both simple and monumental mosques of striking beauty were built in cities of the Islamic world. Despite the borrowings from diverse civilizations, certain common features became characteristic of most mosques and thus serve to distinguish them from the sacred spaces of other religions and cultures.

The most important characteristic of a mosque is that it should be oriented toward Mecca. One or more niches (mihrab) on one of the walls of the mosque often serve as indicators of this direction, called qibla. When the imam leads the prayers he usually faces one of these niches. Next to the mihrab, a pulpit (minbar) is often provided for the delivery of sermons (khutba). Many mosques also have separate areas for performing ritual ablution, and separate sections for women. In many mosques, several rows of columns are used to mark the way for worshipers to line up behind the imam during prayer.

Mosques usually have one or more minarets, or towers, from which the muezzin calls Muslims to prayer five times a day. In addition to their functional use, these minarets have become distinguishing elements of mosque architecture. In large mosques in particular, minarets have the effect of tempering the enormity and magnificence of the domed structure by conveying to the viewer the elevation of divinity above the pretensions of human grandeur.

Most mosques also have a dome, and the line connecting the center of the dome to the niche is supposed to point toward Mecca. Throughout the world there are many mosques that are not actually directed toward Mecca, but such misalignment is due to inaccurate methods for determining the direction of Mecca and does not imply a disregard for this requirement. The mosque is not a self-contained unit, nor is it a symbolic microcosm of the universe, as are some places of worship in other religions. Rather, the mosque is always built as a connection with Mecca, the ultimate home of Muslim worship that metaphorically forms the center of all mosques.

Islamic doctrine emphasizes the oneness, uniqueness, transcendence, and utter otherness of God. As such, God is different from anything that the human senses can perceive or that the human mind can imagine. The God of Islam encompasses all creation, but no mind can fully encompass or grasp him. God, however, is manifest through his creation, and through reflection humankind can easily discern the wisdom and power behind the creation of the world. Because of God's oneness and his transcendence of human experience and knowledge, Islamic law forbids representations of God, the prophets, and among some Muslims, human beings in general. As a result of this belief, Islamic art came to excel in a variety of decorative patterns including leaf shapes later stylized as arabesques, and Arabic script. In modern times the restrictions on creating images of people have been considerably relaxed, but any attitude of worship toward images and icons is strictly forbidden in Islam.

Before Islam, many Arabs believed in a supreme, all-powerful God responsible for creation; however, they also believed in lesser gods. With the coming of Islam, the Arab concept of God was purged of elements of polytheism and turned into a qualitatively different concept of uncompromising belief in one God, or monotheism. The status of the Arabs before Islam is considered to be one of ignorance of God, or jahiliyya, and Islamic sources insist that Islam brought about a complete break from Arab concepts of God and a radical transformation in Arab belief about God.

Islamic doctrine maintains that Islam's monotheism continues that of Judaism and Christianity. However, the Qur'an and Islamic traditions stress the distinctions between Islam and later forms of the two other monotheistic religions. According to Islamic belief, both Moses and Jesus, like others before them, were prophets commissioned by God to preach the essential and eternal message of Islam. The legal codes introduced by these two prophets, the Ten Commandments and the Christian Gospels, took different forms than the Qur'an, but according to Islamic understanding, at the level of doctrine they are the same teaching. The recipients of scriptures are called the people of the book or the "scriptured" people. Like the Jews and the Christians before them, the Muslims became scriptured when God revealed his word to them through a prophet: God revealed the Qur'an to the prophet Muhammad, commanding him to preach it to his people and later to all humanity.

Although Muslims believe that the original messages of Judaism and Christianity were given by God, they also believe that Jews and Christians eventually distorted them. The self-perceived mission of Islam, therefore, has been to restore what Muslims believe is the original monotheistic teaching and to supplant the older legal codes of the Hebrew and Christian traditions with a newer Islamic code of law that corresponds to the evolving conditions of human societies. Thus, for example, Islamic traditions maintain that Jesus was a prophet whose revealed book was the Christian New Testament, and that later Christians distorted the original scripture and inserted into it the claim that Jesus was the son of God. Or to take another example, Muslims maintain that the strict laws communicated by Moses in the Hebrew Bible were appropriate for their time. Later, however, Jesus introduced a code of behavior that stressed spirituality rather than ritual and law.

According to Muslim belief, God sent Muhammad with the last and perfect legal code that balances the spiritual teachings with the law, and thus supplants the Jewish and Christian codes. According to the teachings of Islam, the Islamic code, called Sharia, is the final code, one that will continue to address the needs of humanity in its most developed stages, for all time. The Qur'an mentions 28 pre-Islamic prophets and messengers, and Islamic traditions maintain that God has sent tens of thousands of prophets to various peoples since the beginning of creation. Some of the Qur'anic prophets are familiar from the Hebrew Bible, but others are not mentioned in the Bible and seem to be prophetic figures from pre-Islamic Arabia.

For the Muslim then, Islamic history unfolds a divine scheme from the beginning of creation to the end of time. Creation itself is the realization of God's will in history. Humans are created to worship God, and human history is punctuated with prophets who guarantee that the world is never devoid of knowledge and proper worship of God. The sending of prophets is itself understood within Islam as an act of mercy. God, the creator and sustainer, never abandons his creations, always providing human beings with the guidance they need for their salvation in this world and a world to come after this one. God is just, and his justice requires informing people, through prophets, of how to act and what to believe before he holds them accountable for their actions and beliefs. However, once people receive the teachings of prophets and messengers, God's justice also means that he will punish those who do wrong or do not believe and will reward those who do right and do believe. Despite the primacy of justice as an essential attribute of God, Muslims believe that God's most fundamental attribute is mercy.