Islam

[I]t is often said that Islam was 'spread by the sword'.

This is not the whole story, however. From the time of Muhammad onwards, Muslim agents worked to promote the 'Message of the Prophet'; likewise, recorded instances of conquered peoples being forced to accept Islam are the exception, not the rule...

While conversion to Islam was not discouraged in the first century of conquest, it wasan't greatly encouraged either - largely because Muslims were exempt from the jizya tax. If the newly conquered countries had been converted en masse, the Arab empire would have rapidly drained of revenue. Tolerance was the norm instead, and Christians, Jews and some Zoroastrians became ahl-adh-dhimma - protected peoples.
- Justin Wintle, The Rough Ruide History of Islam, p. 52-53