The use of the articles 'a' and 'an' are related to the noun in which they precede. If the first letter of the noun is a vowel one would use the article 'an', and if the first letter is a consonant, the article 'a' is used. However there are exceptions to the rule, if the first letter of the noun is silent, and the second being a vowel, then 'an' would be used.
Example: We would say, 'an hour' not 'a hour', because the 'h' is silent.
When reading the Bible, the term 'an Hebrew' is used. Is this telling us how the noun 'Hebrew' was pronounced when the King James edition of the Bible was released? The pronunciation being silent in the 'h'.