The Wealth of Nations by Part 5 Chapter 1 Page 218

But the reverence of the people naturally preserves the established forms and ceremonies of religion long after the circumstances which first introduced and rendered them reasonable are no more.

Though Latin, therefore, was no longer understood anywhere by the great body of the people, the whole service of the church still continued to be performed in that language. Two different languages were thus established in Europe, in the same manner as in ancient Egypt; a language of the priests, and a language of the people; a sacred and a profane; a learned and an unlearned language. But it was necessary that the priests should understand something of that sacred and learned language in which they were to officiate; and the study of the Latin language therefore made, from the beginning, an essential part of university education.