How Islam Begin-The Spread of Islam. As the ‘seal of the prophets’ after Muhammad (pbuh) there would be no other prophet. As a political guide Muhammad (pbuh) was succeeded by the al Khulafa' al Rashidun, the four ‘rightly-guided caliphs’ who were chosen from his most loyal companions. It was during the leadership of these caliphs ( 632 – 56 ), and particularly under the second caliph and great statesman ‘Umar ibn al Khattab (634–44), that the conquest of territories outside Arabia began.