Abarim Publications' online Biblical Greek Dictionary
There are two different prepositions spelled εν, namely εν (en), meaning in, and εν (hen), meaning one. The latter is the neutral form of the cardinal number εις (heis), meaning one.
The preposition εν (en) appears as leading element of a colossal array of words. It roughly works the same as the English preposition "en-" (as in words like: entrance, enthuse, enslave), although in Greek it's much more commonly and broadly used than in English. The nature of the action that this particle describes (to or towards) provokes the dative case.
The familiar phrase 'in Christ' employs this particle, and it demonstrates that our belief 'in Christ' does not entertain Christ as the object of our faith but rather the environment in which we perform our faith (we believe in Christ the way we dance in the rain). Our believing is done within the reality of Christ, and the object of our faith is not Christ but rather "all things" (Matthew 11:27, Romans 11:36, 1 Corinthians 13:7, Colossians 1:16-17; see our article on πιστις, pistis, meaning faith, for more on this).
Our particle is used whenever we're in a place or building, on an elevation, amidst a multitude, in someone's presence, at someone's feet, enveloped by something, in a certain time period, in a certain situation, under a certain condition, in a certain way, under a certain influence, with the help of something.
Our preposition generally means in, at or by and expresses a situation either inside something or endowed with something, but always at rest — this in contrast to the more motive prepositions εις, eis, meaning into, and εκ, ek, meaning out of.
In the rare cases in which this preposition is used with verbs of motion, the use of εν (en) implies that we're looking at a situation that results from a motion inward, in stead of the actual movement.