Why are they called sects in Islam and not denominations?

The term "sects" in Islam refers to the divisions that emerged among Muslims after the death of the Prophet Muhammad ﷺ, primarily due to differing interpretations of Islamic teachings and leadership. This is distinct from the term "denominations," which is often used in other religious contexts, such as Christianity, to describe various branches within a single faith that may differ in practices or interpretations but usually remain within a broader, unified tradition.

The Qur'an addresses the issue of division within the faith, emphasizing the importance of unity and warning against factionalism. For instance, it states: Lo! those who disbelieve in Allah and His messengers, and seek to make distinction between Allah and His messengers, and say: We believe in some and disbelieve in others, and seek to choose a way in between 4:150. This highlights the seriousness of splitting into factions and the consequences of not adhering to a unified belief.

Additionally, the Prophet ﷺ remarked on this division, indicating that his community would also face fragmentation. He stated: The Jews split into seventy-one or seventy-two sects, and the Christians likewise. My nation will be divided into seventy-three sects Sunan al-Tirmidhi 40:35SunniHasan Sahih (per Al-Albani). This narration illustrates that, similarly to the followers of previous prophets, the Muslim community would also experience division, which is why the term "sects" is employed in this context.

Scholars have discussed the implications of these divisions. Ibn Kathir notes that the divisions arose after knowledge had come to the people, which led to disputes and enmity among them (Tafsir Ibn Kathir). Meanwhile, Majma al-Bayan highlights that different sects deviated from the original teachings of Islam, which were meant to uphold unity in worshiping Allah alone.

In summary, the use of "sects" in Islam reflects a significant historical and doctrinal reality of division among Muslims, rooted in both the Qur'an and the teachings of the Prophet ﷺ, distinguishing it from the notion of "denominations" in other faiths.