What is known about religion in the subcontinent comes through a transmission unlike any other in this atlas, and the shape of the record is worth understanding before the content.
The Vedic hymns were preserved by recitation for many centuries before anyone wrote them down, and they were preserved with a precision that is genuinely remarkable. Brahmin schools developed layered recitation methods that repeat the text in permuted orders, so that a single error in word or accent would break the pattern audibly and be corrected. The result is a text that specialists regard as transmitted with less corruption than most manuscript traditions of comparable age achieve on parchment.
The manuscripts are a different matter. Palm leaf and birch bark do not survive the climate, so copies were made afresh every few generations and the physical witnesses to even very old texts are usually recent. Dating therefore rests on language and internal evidence rather than on a datable object.
The archaeological record adds a gap that may never close. The Indus Valley cities produce seals and figurines that look religious, and the script on them remains undeciphered despite a century of effort, so their beliefs are read from images alone. The proposals connecting them to later Hindu practice are arguments from resemblance.
Buddhist and Jain material survives on inscription and in Pali and Prakrit canons preserved outside the subcontinent as well as within it, which is why the transmission history of each has its own geography.