Ἐν + dative

Prepositions are some of those tiny words that are incredibly meaningful to a text/passage. The danger is heading to a lexicon (dictionary), seeing the 10-15 possible meanings and randomly selecting one that best fits our idea of what the word should mean. 

By looking at a preposition's more likely meaning when paired with a particular case (i.e., with its object), the number of options dwindle significantly. Furthermore, the semantic range of main verbs (or head noun) often limits the scope of the preposition as well. 

Wallace also notes how whether a verb is transitive, intransitive, or stative can affect the function of the preposition (Grammar, 359). 

Ἐν is only used with the dative case. (Keep in mind, the reader should look for the main verb to help narrow down these options.)

For more detailed discussion of this preposition and case, see Wallace, Grammar, 372-75/

1.    Location: Answers where action is occurring (could be literal or more figurative, such as "in Christ")

2.    Temporal: When an event or action is occurring (distinct from genitive and accusative, in that it gives a specific time)

3.    Association: Indicates personal relationship or with whom something is done

4.    Cause: The basis of which an action/event is done

5.    Means/Instrument: How the main verb is accomplished

6.    Reference/Respect: A caveat or specific idea that makes the main idea true ("with regard to")

7.    Manner: Characteristics or attitudes of how an action should be accomplished (e.g., with joy)

8.    Possession: Lists items that someone owns

9.    Standard/Rule: Provides the measure by which a statement or action is judged

10. Could be equivalent to εις with a verb of motion

Some argue for a dative of agency in the NT, but Wallace points out that there should be certain defining characteristics of "agency," namely that the acting agent is personal and (without a doubt) performing the main action. With these two qualifiers in place, there are no clear examples of a prepositional dative of agency in the NT. (There may be a dative case noun of agency in Luke 23:15). More likely, the dative in these instances is functioning as means/instrument. (Admittedly, sometimes the "instrument" is personal, as in the Holy Spirit, see Mark 1:8, 1 Cor 12:13, and the passage below).

One can clearly see that "content" is not listed above, which affects a beloved passage in Ephesians 5:18. The simple dative may indicate content, but with this preposition, it is highly unlikely. In Ephesians 5:18, the verb of filling would much more likely take a genitive of content if that's what Paul was trying to communicate. So how is the dative being used?

"Filled by the Spirit" = Means/Instrument

Paul uses "fill" language throughout Ephesians, and the genitive of content is used in his major prayer. We are to be filled with the knowledge of God. The dative here seems to suggest that the Holy Spirit is the instrument used to accomplish this purpose. See my discussion of the passage as a whole for more background.