How did Muhammad become a Messenger of God?

At the age of 40, while engaged in a meditative retreat, Muhammad
received his first revelation from God through the Archangel Gabriel.
This revelation, which continued for twenty three years, is known as
the Qur'an
Muhammad began to share the revelations he received from God with the
people of Makkah. They were idol worshippers, and rejected Muhammad's
call to worship only One God. They opposed Muhammad and his small
group of followers in every way. These early Muslims suffered bitter
persecution.
In 622, God gave the Muslim community the command to emigrate. This
event, the hijrah or migration, in which they left Makkah for the city
of Madinah, some 260 miles to the North, marks the beginning of the
Muslim calendar.
Madinah provided Muhammad and the Muslims a safe and nurturing haven
in which the Muslim community grew. After several years, the Prophet
and his followers returned to Makkah and forgave their enemies. Then,
turning their attention to the Ka'bah (the sanctuary that Abraham
built), they removed the idols and rededicated it to the worship of
the One God. Before the Prophet died at the age of 63, most of the
people of Arabia had embraced his message. In less than a century,
Islam had spread to Spain in the west, as far east as China.