Usul al-din 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 [prophet hood of the Prophet Muhammad (s)], and resurrection are the three main principles of Islam. All Islamic sects believe in these three principles; And For 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 discussion over the sufficiency of believing in the principles of religion out of emulation (taqlid).

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" , principles of beliefs), "usul al-iman", principles of the faith), "ummahat 'aqa'id imani" , the main faith-based beliefs), and "usul Islami" ( 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 Ayatollah 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 prophet hood 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.

Principles of a Religious Denomination

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.

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.