Final answer:
Muhammad and his followers left Mecca in 622 CE because they were forced out by the citizens who did not agree with Muhammad's new messages of social responsibility and his claim of monotheism. They found refuge in the nearby city of Medina. This event, known as the hejira, marks the start of the Islamic calendar.
Step-by-step explanation:
In 622 CE, Muhammad and his followers left Mecca because they were forced out by the citizens who did not agree with Muhammad's new messages of social responsibility and his claim of monotheism. They found refuge in the nearby city of Medina, where they were welcomed by the people who hoped that Muhammad could mediate disputes between clans and families. This journey, known as the hejira, marked the start of the Islamic calendar.