My parents had a grapevine in the backyard. They just let it grow and some of the branches grew up and intertwined around this big evergreen tree about 20 feet in the air, others grew through the cedar hedge that was beside it. Some of these branches must have been forty feet long, but they never produced any grapes.
In fact the only grapes I ever saw on this plant were hidden underneath, out of the sun very close the original vine. But they were very small and sour because they didn’t get much light, and these long fruitless branches probably took most of the water and nutrients to keep themselves alive and growing fruitlessly. As I read this verse it occurred to me that these long branches seemed to be trying to get away, to not abide in the vine, but were reaching, searching for something else.
They were attached to the vine but straying away in search of something else, to fulfill some other purpose, because it was certainly not to produce fruit. Only the ones that stayed close to the vine produced any fruit, but even the quality of their fruit was affected by the other branches that were wandering off.
These rogue branches do not represent nonbelievers, but believers who wander away, do not abide in Jesus, and do not produce fruit. This is confirmed by what he says in the first verse of the next chapter, “I have said all these things to keep you from falling away.”