We know and understand the meaning of the word unless. It expresses an inevitable law, something which cannot, may not, be dispensed with. "As the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in me." There is but one way for the branch to bring forth fruit; there is no other possible means. It must abide in the unbroken communion with the vine. Why would Christ go so far as to add these words of warning? Why would words of warning be necessary?
Because He knows fully the meaning, the intention, of the words: "Abide in Me." He is aware of the call for the denial of self found in those three simple words. He knows how easy it is to give in to the temptation of thinking we are able to bear fruit out of our own efforts. He know how difficult it is for us to accept the belief that continuously abiding in Him is not a maybe, but a necessity.
Does this mean that I must be totally, absolutely absorbed in Christ, and Christ alone, just as the branch abides in the vine? Yes, we must. His words, "unless you abide"; "unless it abides" and "neither can you" have no possible exceptions. There is no bargaining, it is either, or; no exceptions. If we are to bear fruit, if we are to be what Christ would have us to be, our whole being must be completely and totally devoted to abiding in Him as that of the natural branch abiding in the vine.
Abiding in the vine then becomes nothing more, or less, than the restful surrender of the soul, the self, the very being, to let Christ have all and work all, allowing the Christ in you to care for the Christ in the other unconditionally.