Prophet Muhammad (peace be upon him) began preaching Islam in two main phases: an initial three years of private, secret calls to close relations and friends, followed by a public proclamation to the people of Mecca. 

The First Revelation (c. 610 CE)

Muhammad was around 40 years old and accustomed to meditating in the Cave of Hira on Mount Jabal an-Nour, near Mecca. During one of these retreats, the Archangel Gabriel (Jibril in Arabic) appeared to him and commanded him to “Recite!” (Iqra in Arabic) the first verses of the Qur’an: 

“Read! In the Name of your Lord, Who has created, created man from a clot. Read! And your Lord is the Most Generous—Who taught by the pen, taught man what he knew not.” (Qur’an 96:1-5) 

Overwhelmed, Muhammad returned home and was consoled by his wife, Khadijah, who was the first person to believe in his message. 

The Secret Phase of Preaching (c. 610–613 CE)

For the first three years, the message of Islam was spread discreetly. This approach was likely a strategic choice to build a small, strong foundation of believers before facing the inevitable opposition from the polytheistic Meccans, who had strong economic and social ties to their traditional idol worship. 

The earliest converts included:

• Khadijah bint Khuwaylid: His wife, the first believer.
• Ali ibn Abi Talib: His young cousin, who was about ten years old at the time.
• Zayd ibn Harithah: His freed slave and adopted son.
• Abu Bakr al-Siddiq: A close friend and wealthy merchant, who used his influence to bring others to the faith. 

During this period, the small community practiced their faith in private, focusing on the core message of monotheism and social justice. 

The Public Proclamation (c. 613 CE onwards)

After approximately three years, divine revelations commanded Muhammad to proclaim the message of Islam openly. He began by calling his relatives and then the general populace of Mecca to Mount Safa, challenging their practice of idol worship and calling them to the worship of one God, Allah. 

This public call led to mockery, verbal abuse, and severe persecution of Muhammad and his followers by the ruling Quraysh tribe. The situation eventually forced the Muslim community to emigrate to the city of Medina (then known as Yathrib) in 622 CE, an event known as the Hijra, which marks the beginning of the Islamic calendar. In Medina, Muhammad established the first Muslim community and state, from where Islam grew into a powerful movement that eventually led to the peaceful conquest of Mecca and the spread of Islam across the Arabian Peninsula.