Fishman's generally non-biased, generally accurate piece of Hackery:
The Caliphates and the other Muslim powers (Mughals, Ottomans, Persians etc.) were spread by the sword, quite obviously. But so were all the other Empires in history. The Persian and Roman Empires had been expanding violently for centuries and the Western European countries were the descendants of the Germanic tribes that had crushed Rome in the fifth century.
Islam as a religion was not really spread by the sword except in parts of India and the Balkans. The Ummayads actually made mass conversions forbidden because they didn't want the privilaged status of the Arabs to be undermined. In most of the Caliphate, Islam stayed a minority religion until sometime during the Abassid rule.
Like Christianity, Islam also spread beyond the bounds of Islamic states. this mainly happened in tribal, 'unclivilised' areas, another similarity to Christianity. These areas were sub-Saharan Africa, Central Asia and Southeast Asia. Central Asian Islam is quite unusual as the regions it was present in were conquered by shamanist Mongols, who later converted to Islam and helped it spread further.
With the exception of Southeast Asia, however, the interaction between Islamic states and these areas was certainly not peaceful, as they were frequently being raided for slaves. Slave traders who entered these areas tended to bring Islam with them and spread it there. This somewhat resembles Christianity's spread in Latin America and west Africa, but it is a bit shakey.
Islam also spread in the west indies and south America before the modern era. This was because many African slaves taken by Europeans were Muslim, and some retained their religion after being enslaved. There is not really any equivalent of this in Islamic history.
Pre-modern Christian countries have rarely conquered non-Christian areas with strong organised religions. The few examples are the Reconquista in Spain, the Spanish conquest of the Aztecs and Incas, the Russian conquest of Siberia and the Crusades. In the first three examples, non-Christian religions were crushed almost immediately. In the Crusades, Muslim populations did remain, but Christians were still a very large minority in the region anyway.
In the Russian conquest of Siberia (specifically western Siberia) the government tried to Christianise the native population, which was to some extent successful. However, some regions remain Muslim today.
This shows that Christian countries were a bit more prone to forceful religious conversion than Islamic ones. However, all of the major forced conversions were commited by the Spanish, so maybe this shows more about Spain than Christianity in general.
