Joh 15:4 Stay on me, and I in you. Just as a shoot is unable to produce fruit by itself unless it remains on the vine, neither can you unless you stay on me.
Joh 15:5 I am the vine; you are the shoots. The one who stays on me and I in him produces much fruit because you can do nothing without me.
Joh 15:6 If anyone does not stay on me, he is thrown out like that detached shoot and he dries up. They gather them, throw them into the fire, and they are burned up.
The consequences of separation
I have to be a bit inconsistent with the preposition ἐν here, because Jesus is talking about a reciprocal relationship, but still keeping up the vine/shoot metaphor. So, “on” is appropriate, because shoots can only produce grapes if they stay on the vine. This is an example of what Danker alludes to “with numerous other resources in Engl. to express contextual nuances of ἐν: at, on, among, near, with, by.”
Jesus also refers to final punishment in Gehenna here by saying that detached shoots are gathered, thrown into the fire and burned up (καίω). While the word can refer to the mere lighting or kindling of a fire, in this reference it is to a consuming fire which ultimately produces the second death (Revelation 21:8).
Some people have convinced themselves that final punishment merely means being thrown out – separated from the presence of God forever. Jesus shows here that as a consequence of this separation, the lost will also be gathered up, thrown into the fire, and burned up. Hell is a terrible place, but the only way to escape it is to stay on the vine – stay in Jesus.