The spread of Islam saw the conquest of tribal/pastoral societies by a much more complex and organised civilisation. I found a pretty good quote on Wikipedia that holds that conversion to Islam "represented the response of a tribal, pastoral population to the need for a larger framework for political and economic integration, a more stable state, and a more imaginative and encompassing moral vision to cope with the problems of a tumultuous society."

If you can rationalise wars of conquest being relatively ‘peaceful’ paths to conversion, much more oppressive methods of subjugation and subordination - forced conversion, ethnic cleansing etc. - would come later.

From what I understand, in the early stages of the caliphate Islam was a status symbol that separated the ruling Arab elite from the masses.