Why were Muslims divided into multiple sects and schools of thought?

Islam is a monotheistic religion that calls for people to unite and reject division and discord. The Quran and Sunnah have warned against disunity and disagreement. The Prophet, peace be upon him, clarified that the Ummah would differ, and he advised adherence to the Sunnah and abandonment of innovations. Islam is not divided into sects; rather, differences among people stem from their moving away from religion and following their whims. Ahl al-Sunnah, or Salafis, are those who adhere to the Book of Allah, the Sunnah of His Prophet, and the guidance of the righteous predecessors, and they prioritize sacred texts over intellectual opinions.