This scene of Mohammed preaching in a mosque was very popular and often depicted in Islamic art - I've posted a version before but this one is better, and it never does any harm to point out that the version of Islam that Allah dictated to Mohammed is very different to the nonsense version being advocated today by some fundamentalists.
When he turned 40 Mohammed began to have what could be described as a mid-life crisis. He was dissatisfied with the world he saw around him, and started to take time out to contemplate in solitude. Much of this was done on Hira, on the night of the 26th Ramadan 610 the angel Gabriel came to him on this mountain and announced that God had chosen Mohammed to be his messenger, to spread his Word and monotheism in the region. The first revelations made by Allah were that there was only one God, and that those who believed in him could attain Paradise as there was an afterlife.
From then on God regularly revealed his message to Mohammed, and this was in turn written down in the Koran. The Koran is unchanging, but the laws of the Muslims were codified into a separate book, the Sharia, which could be changed through scholarly debate and the will of the people.
Mohammed used persuasion to convert rather than the sword or threats, and this worked on my different peoples, as is shown by the wide diversity of costumes and physiognomy of those listening to him in the mosque depicted here. He began to preach after the first revelation, first converting his wife, then friends, family and others around him.
This manuscript page comes from a copy of the al-âthâr al-bâqiya (The Remaining Signs of Past Centuries; Arabic الآثار الباقية عن القرون الخالية) of Al-Bîrûnî (d. 1048), created in Iran in the 16th century. (Image: BNF)