There are two issues here: the translation of the noun and the proper translation of the article. The noun is αποστασια. Although it could mean “rebellion,” the most literal translation is “departing” or “falling away.” Αποστασια, derived from the words “απο (away from)” and “στασισ (standing),” means “away from standing” or “falling away.” “Rebellion” is the figurative meaning, since the act of rebellion is due to “falling away” from authority (whether it be the truth or a government). Although it is common to interpret this “falling away” as the end-time rebellion, the literal translation enables the reader to interpret “falling away” as referring to the departing of believers from earth in the rapture. In a chapter as mysterious and ambiguous as 2 Thessalonians 2 it may be wise not to translate anything too narrowly in a way that eliminates the possibility of various valid interpretations.
In regards to the article, αποστασια is preceded by “η,” the definite article. The definite article would normally be translated as “the.” However, the KJV reads “a falling away.” This is because the use of the article here is to substantivise the verb-like noun “αποστασια.” Αποστασια is without a doubt a noun, but it has the connotation of a participle verb. It means, “falling away,” which in English is a gerund, a verb-turned-noun. Without the article, the passage would read in the sense of “that day shall not come, except falling away comes first.” The substantiver makes it clearer that “falling away” is an event (noun) rather than a generic act of falling away (verb). Since there is no indefinite article in Greek, the best way to substantivise the noun and give it greater force as a noun is to precede the verb-like-noun with a definite article. Since the definite article here is neither anaphoric nor kataphoric (referring to something previous or following) nor deictic/demonstrative (pointing to something), there is no need to translate the article as the definite article in English. English has the indefinite article which serves a substantivising function, so “a falling away” is a valid translation.
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