Q:    Regarding John 15:4, what exactly does “abide” mean?

A:    "To remain in" or "to dwell in."

Q:    Is that the same as being in Christ?  

A:    Well, almost.  To be "in Christ" means exactly that.  To dwell or abide in someone is pretty much the same thing but emphasizes relationship whereas "in Christ" shows location.

Q:    Does that mean we can be "in Christ" but still not produce fruit?

A:    No; but it takes time to produce fruit that is evident.  Just the growth process is fruit.
The production of fruit comes from the Holy Spirit and if a person has the Holy Spirit, that person has to be producing fruit.

Q:    Then why the different words?  Why does he say "abide in me"?  If we're saved we are abiding in him, right?  Is he talking to believers and non-believers?

A:    In this chapter He is using the vine as a teaching tool.  In the Old Testament, the vine was Israel.  In this chapter He is saying that if a person (Jew) considers himself to be saved (on the vine) then he will be abiding in the vine and fruit will be produced from the vine through his branches.  If the branch does not produce fruit, that means it is attached physically but is cut off from the source coming through the vine stalk and therefore has no life in it.  The keeper of the vineyard will cut off that branch because it is dead and cannot produce fruit.  So "abide" has to do with being one with the source coming through the vine, not just physical attachment.  Physical attachment would represent a Jew being physically involved with Judaism but not having the life of Christ in him.

Q:    So then when he says "without me you can do nothing" is he still talking to Jews or is that for all of us?

A:    He is telling them this is for everyone (everyone must abide in Him) but in particular in reference to the Jews as here.  In Romans 11:11-36 Paul talked about the Gentiles being grafted into the olive tree (another symbol used of Israel in the Old Testament).