What happens to a disciple that stops abiding in Him in John 15:6?

That portion of the passage reads as follows,

Joh 15:5-6 I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing. (6) If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.

We must remember that Jesus only taught Jews during His earthly ministry, Mat 15:24, so, this passage does not apply to believers saved by the grace gospel. The general context of John chapters 13–16 is that Jesus is in the upper room the evening before His arrest and trial. After the Last Supper, Jesus speaks these words to the disciples, the believing remnant of Israel. So, keeping this context in mind, let's take a closer look at verse 6.

Verse 6 is a warning to the people to whom the Apostles will minister, not to the Israelite believers. The Apostles must warn the Jews who are still unsaved about the dangers of not abiding in Christ. Those who do not believe, will wither like a broken branch, and be thrown into the fire and burned. This is the all-consuming fire of Jesus’ second coming that will later give way to the flames of Hell and the Lake of Fire, (see Matt 13:30, 37-43 and 2Thess 1:7-9).

Thus, John 15:6 poses no threat to believers, (be it Christians in the Dispensation of Grace or the Messianic Jews of that day). It has nothing to do with working for one's salvation. Nothing in this passage speaks of someone losing their justification, salvation, or eternal life. When the Apostles start their ministries in the Book of Acts, it is a warning to those unbelieving Jews who have not joined the Little Flock, Israel's believing remnant. They will suffer God's wrath if they do not abandon apostate Israel and become a part of redeemed Israel, (see Zechariah 13:8–9).