Al-Biruni's engagement with comparative religion did not begin with India. Decades earlier, still in his twenties, he completed Al-Athar al-Baqiya, The Chronology of Ancient Nations, a systematic study of the calendars, eras, festivals and chronological systems of numerous religious communities he had direct or indirect access to in Central Asia and the wider Islamic world, including Zoroastrians, Jews, Christians, Sabians and various sects then still active around Khwarezm, many of which are known today chiefly or only through his description. That earlier work shares the comparative, documentary instinct that later shaped the Kitab al-Hind: al-Biruni is less interested in ranking religions than in recording precisely how each community reckons and marks time, treating calendrical and festival practice as a serious object of scholarly attention in its own right rather than incidental detail. This religious and historical writing sat alongside a scientific output remarkable even among Islamic Golden Age polymaths, including a reasoned estimate of the Earth's circumference calculated from a single mountaintop observation, extensive work in mineralogy and pharmacology, and astronomical writing that engaged seriously with the possibility of the Earth's rotation centuries before it became established European science.
Modern assessments of al-Biruni consistently single out the combination that makes him unusual even within a period rich in polymaths: a working scientist's insistence on direct observation and primary source verification applied, with the same rigor, to religions he did not practice himself, a method the Encyclopaedia of Islam and subsequent scholarship both treat as placing him well ahead of most premodern writing about religious difference.