A sacred language is often the language which was spoken (and written) in the society in which a religion's sacred texts were first set down; however, thereafter these texts, becoming fixed and holy, remain frozen and immune to later linguistic developments.
In some instances, the sacred language may not even be (or have been) native to a local population, that is, missionaries or pilgrims may carry the sacred language to peoples who never spoke it, and to whom it is an altogether alien language.