The Relación de las fábulas y ritos de los Incas exists because a bishop wanted an operational description of what his clergy were up against. Written around 1575 for Sebastián de Lartaún, it is the closest thing there is to a systematic account of the Inca ceremonial year: the great festivals in their months, the Situa purification with its running of torches and its washing in the rivers, the Capac Raymi initiation of young men of the ruling lineages, the offerings sent out along the shrine lines from Cuzco, and the capacocha, in which children were taken from the provinces to the capital and then returned to high places to be given.
Molina had been in Cuzco a long time, preached in Quechua, and questioned men old enough to have taken part before 1532. He records prayers in Quechua rather than only in Spanish paraphrase, which is unusual and valuable, because the Spanish paraphrases of the period habitually reshape the theology. The purpose was suppression. Nothing in the text pretends otherwise. But a hostile witness who takes the trouble to get the details right is more use than a sympathetic one who does not, and the archaeological work on the shrine lines around Cuzco has repeatedly found his descriptions to be usable.