How do we differentiate sacred and secular literary text?[in detail]
Sacred is referring more to the church. In music history, it deals with the period of time after Gregorian chant when music was written for the church services in the Proper and the Ordinary including parts such as: Kyrie, Conductus, Antiphon...etc. Stylistically, it is more plain and goes more with the syllables of the text in the chant. There is often a lot of short phrase in repeated metrical patterns.
Secular music is the more mainstream "non-church" music. Stylistically, it includes longer drawn out phrases. It uses a lot of expression to provide a mood with the music.