Uṣūl al-dīn (Arabic: أصول الدین) or principles of religion or roots of faith are a set of essential beliefs in Islam that every Muslim needs to believe in; otherwise, he would not be considered a Muslim. Tawhid (oneness of God), nubuwwat (prophethood of the Prophet Muhammad (s)), and resurrection are the three main principles of Islam. All Islamic sects believe in these three principles; but each one has special principles which separate it from other sects. For the Shi'a, 'adl (justice of God) and imamate are the additional principles.
Believing in the principles of religion is obligatory; but there is disagreement as to whether certainty of the principles is required or supposition is enough. Also there is a discussion over the sufficiency of believing in the principles of religion out of emulation.
Most religious scholars believe that emulation is not permissible in usul al-din and everyone needs to think about these principles for himself and accept them with certainty.
"Usul al-din" is a theological jargon referring to the fundamental beliefs of Islam the belief in which is required for being considered as a Muslim. Such beliefs are called "usul al-din" (principles or foundations of the religion) because Islamic disciplines, such as fiqh, usul al-fiqh, the exegesis of the Qur'an, and hadith, are based on them. The word, "usul al-din", is contrasted to "furu' al-din" (ancillaries of the religion) which refers to the practical rulings of the religion.
Muslim theologians have referred to the principles of the religion in different ways such as "usul al-i'tiqadat" (Arabic: أصول إعتقادات, principles of beliefs), "usul al-iman" (Arabic: أصول إیمان, principles of the faith), "ummahat 'aqa'id imani" (Arabic: أمهات عقائد إيماني, the main faith-based beliefs), and "usul Islami" (Arabic: أصول الإسلامي, Islamic principles), and so on. However, what they meant to refer with these terms was not restricted to what are taken today as principles of Islam. In many cases, ethical and jurisprudential issues were also referred to as the principles of the religion. For example, al-Ghazali referred to all theological as well as many jurisprudential and ethical issues as the principles of the religion. He considered the prayer and piety as principles of the religion, along with monotheism.
According to Misbah Yazdi, "usul al-din" is a conventional term which can be used in different meanings. In one convention, it refers to all belief-related propositions of the religion. In another convention, it refers to the fundamental propositions of divine religions or a specific religion. It might as well refer to the principles of a religious denomination; for example, one can say that, for the Shi'a, the principles of the religion consist in monotheism, the prophethood of the Prophet Muhammad (s), the Resurrection, justice, and imamate, just as Morteza Motahhari has considered these five beliefs to be the principles of the religion according to the Shi'a.
The principles of Islam consist in monotheism, the prophethood of the Prophet Muhammad (s), and the Resurrection. Anyone who believes in these three doctrines is considered to be a Muslim, and anyone who does not believe in any of these doctrines is not considered as a Muslim. However, there are disagreements among Muslims with respect to the details of these principles. For instance, the majority of Shiite and Mu'tazila theologians believe in the identification of God's Attributes with His Essence, but Ash'aris maintain that God's Attributes are over and above, and external to, His Essence.
Morteza Motahhari emphasizes that the Imams (a) did not coin the term, "usul al-din", and says: it was coined by scholars in order to indicate the main goals of the religion, for there are many other beliefs in Islam in which a Muslim has to believe, such as the belief in angels and the essentials of the religion, such as prayer and fast.
In the past, no particular criterion was specified for when beliefs count as the principles of a religion or a religious denomination. The significance of an issue in a certain period usually led to its consideration as a principle, such as the problem of Divine Justice that has no privilege over other Divine Attributes, but because of major disagreements between the Shi'a and the Mu'tazila, on the one hand, and the Ash'aris—who constituted the majority of Sunni Muslims—on the other hand, it came to be considered as a principle of the religion for the Shi'a and the Mu'tazila.
In addition to the principles of the religion shared by various Islamic denominations, every Islamic denomination involves its own fundamental beliefs that are called the "principles of the denomination". For example, Imami Shi'as believe, in addition to the above three principles, in two additional principles: justice and imamate. Therefore, the principles of the Shiite denomination are five. Issues, such as enjoining the right and forbidding the wrong and justice, count as principles of the Mu'tazila denomination.
The term, "usul al-din," was not used in the Qur'an and hadiths, and it was coined by Muslims. It is conjectured that the use of the term, "al-usul al-khamsa" (the five principles), by Abu l-Hudhayl al-'Allaf, a Mu'tazili theologian, paved the ground for the use of the word, "usul al-din". Since Ibn al-Nadim has attributed a book with the title, "usul al-din", to Abu Musa Murdar, some people think that the term was common in early 3rd/9th century.
However, in some hadiths from the Prophet (s) and the Imams (a), some religious elements were introduced as tenets of the religion. For example, Sunni Muslims have transmitted a hadith from the Prophet (s) to the effect that Islam is founded on five elements: monotheism, the prophethood of Muhammad (s), the saying of the prayer, paying the zakat, hajj, and fasting in the Ramadan month. In al-Kafi, Kulayni has narrated a hadith from Imam al-Baqir (a) that Islam has been built upon five elements: prayer, zakat, fasting, hajj and wilaya and wilaya is superior to other elements. Moreover, in response to a question about things without which faith is not accepted, Imam al-Sadiq (a) mentioned testifying to the oneness of God, belief in the prophethood of the Holy Prophet (a) and what he has brought from God, belief in zakat and accepting the wilaya of Ahl al-Bayt (a). In this hadith, the narrator has referred to these elements as the pillars of Islam.
The majority of scholars require certainty with respect to the principles of the religion. In his book, al-Bab al-hadi 'ashar, al-'Allama al-Hilli claimed that scholars had consensus over this requirement, but al-Shaykh al-Ansari cited other views in this regard as well, such as the view according to which it is sufficient to presume, or have probabilistic knowledge (zann) of, the principles of the religion. According to al-Shaykh al-Ansari, it is implied by what is quoted from al-Muhaqqiq al-Ardabili and his student, Sahib al-Madarik, as well as by the writings of al-Shaykh al-Baha'i, al-'Allama al-Majlisi, and Fayd Kashani, that if a person merely presumes the principles of the religion, then he or she counts as a Muslim.
Al-Shaykh al-Ansari himself holds, in his al-Rasa'il, that the mere presumption of the principles of the religion is not sufficient for counting as a believer, and that given many hadiths implying the requirement of knowledge, one is obliged to inquire about such principles in order to achieve certainty, if possible. According to al-Shaykh al-Ansari, a person who has not achieved certainty is not a believer, because according to hadiths, knowledge is required for faith. However, such a person does not count as an unbeliever either, because there are many hadiths implying that some Muslims are neither believers, nor unbelievers.
According to al-Shahid al-Thani in Haqa'iq al-iman, almost all Muslim scholars believe that it is not permissible to follow other people in the principles of the religion, because the belief in such principles should be with certainty, but following other people does not provide one with certainty.
Al-Shaykh al-Ansari has reported that the majority of scholars believe that it is obligatory to inquire about and argue for the principles of the religion. However, there are other views here as well. For example, some scholars hold that taqlid or following other people with respect to the principles of the religion is permissible if it yields knowledge. Al-Shaykh al-Ansari himself believes that taqlid in Islamic principles is permissible, because what is required for faith by hadiths is knowledge, and not inquiry. Moreover, because of many doubts about the principles of the belief, one cannot obtain certainty even after a great deal of inquiry. Al-Shaykh al-Tusi also believes that a person who cannot personally inquire about the principles of the religion can follow a scholar in this regard.
Today, the first issue that appears in books of tawdih al-masa'il (or manuals of Islamic laws) is taqlid in the principles of the religion. According to the fatwas of marja's, a Muslim should believe in the principles of the religion with certainty and on the basis of arguments. However, some marja's, such as Sayyid Muhammad Rida Gulpayigani, Sayyid 'Ali Sistani, and Lutf Allah Safi Gulpayigani, have added that if one obtains certainty in the principles of the religion, even without any arguments or reasoning, then they count as Muslims.
In most of the theological works a chapter is dedicated to the discussion of usul al-din. However, considering the importance of the isuue, there are works dedicated to usul al-din. Some of the most important of such works is as follows,