Quechua names both a family of Andean languages and the peoples who speak them, and the two are not the same thing. The family was already spread along the Andes before the Inca, whose state adopted a variety of it as an administrative language and carried it well beyond its earlier range. Willem Adelaar and Pieter Muysken's survey sets out the internal divisions, which are wide enough that speakers of the northern and southern branches cannot understand one another.
The colonial church extended it further still. Quechua was treated as a general language for missionary use, catechisms, confession manuals and sermon collections were printed in it from the sixteenth century, and priests were required to learn it, so a Christian vocabulary was built inside the language. That is why Quechua words for sin, soul and prayer are a mixture of borrowed Spanish and repurposed Andean terms, and the repurposing carried older meanings along with it.
After the Tupac Amaru rebellion of 1780 the colonial authorities moved against the language, and against Inca dress and titles with it. It survived in the countryside, and it is spoken today by several million people, with official standing in Peru and Bolivia and a growing written literature of its own.