Due to modern standards, the old-fashioned English word "heathen" is indeed an offensive one which has a rather ancient beginning of existence.

In the Merriam-Webster's 11th Collegiate Dictionary, its noun form has got two meanings:

1: an unconverted member of a people or nation that does not acknowledge the God of the Bible
2: an uncivilized or irreligious person

Essentially, the doctrine reflected in the word dictates that the civilization and education of a person equals to the enlightenment of him about the great existence, authority, and love of God. Well, that had been true in Western history when education and civilization had been mainly conducted in and through Christian churches and their affiliated facilities.