Why must Islamic believers travel to Mecca at least once in their lives?

Islamic believers are required to travel to Mecca at least once in their lives because it is one of the Five Pillars of Islam known as the Hajj. The Five Pillars are fundamental acts of worship and the foundation of a Muslim's faith and practices. The Hajj is a duty that must be undertaken by all adult Muslims who are physically and financially capable of making the journey, at least once in their lifetime.

The pilgrimage to Mecca demonstrates the solidarity of the Muslim people, and their submission to Allah (God in Arabic). The rites of the Hajj include visiting the Kaaba, which is considered the "House of Allah" and is located in the Sacred Mosque of Mecca (Masjid al-Haram), and performing acts of devotion and rituals that trace back to the Prophet Abraham (Ibrahim), his wife Hagar (Hajar), and his son Ishmael (Ismail).