Complex sentence: At a lower socioeconomic level, most new converts to Islam were able to convert their holdings into freehold farms; serfs, who constituted the majority of the population, were predominantly Christians.
Split into simpler sentences: At a lower socioeconomic level, most new converts to Islam were able to turn their holdings into freehold farms. At the bottom of the socioeconomic ladder was the serfs, who constituted the majority of the population and were predominantly Christians.