What is the reason for the doctrinal divisions in Islam, and how did Muslims maintain their unity despite the differences in their sects?

The Prophet (peace and blessings be upon him) clarified the evidence to his Ummah and did not die until Allah perfected the religion. The Companions and the Tabi'un followed this approach, but after the three favored centuries, blameworthy disagreements emerged due to internal factors such as following whims and ignorance, and external factors such as mixing with other nations. Despite the emergence of sects and innovations, these groups remained a discordant voice and were contained, and the majority of Muslims remained on the Sunni path, as scholars fought innovations. The Prophet (peace and blessings be upon him) foretold the occurrence of this disagreement, warned against it, commanded adherence to the Muslim community, and gave good tidings of the existence of a group upon the truth. The Muslims' continued unity despite conspiracies is due to the religion being preserved by Allah Almighty's preservation.