The evidence for Tengrism is unusually front-loaded: the oldest sources are the most explicit, and much of what follows is later reconstruction.
The Orkhon inscriptions are the foundation. Carved in the Mongolian steppe in the early eighth century, in a runiform script, for rulers of the second Turkic khaganate, they are the earliest substantial texts in any Turkic language and they state a theology of heaven, earth and rule directly, in the first person, as royal address. They were deciphered at the end of the nineteenth century.
The Secret History of the Mongols, from the thirteenth century, supplies the fullest narrative source, an internal account of the rise of Chinggis Khan preserved in an unusual way: the Mongolian text survives chiefly through Chinese transcription made for the purpose of teaching the language.
Chinese dynastic histories record the steppe peoples from the outside, over many centuries, with the interests of a settled empire describing its neighbours. Persian and Arabic historians add another external layer, and the reports of European travellers and envoys to the Mongol courts a third.
Modern Tengrism complicates the picture rather than clarifying it. Its revival since the 1990s has produced a substantial literature that is partly scholarship and partly national construction, and reading current statements of the religion back onto the eighth century is a mistake that is easy to make.