John 15:6

If a man abides not in me, he is cast forth as a branch, and is withered; and they gather them, and cast them into the fire, and they are burned.

Here is the answer to the old question of whether or not a Christian can so sin as to be lost. Both the precept and the example are here. Judas, at first a true apostle, did not abide in Christ and was cast forth as a branch. Salvation for every man ever born turns upon just two questions: (1) is he "in Christ"? and (2) does he remain "in Christ"? Given an affirmative answer to those two key questions, a man's salvation is absolutely secure. There is no way to be lost if one, "being in Christ." remains "in him" until probation is ended. "Blessed are the dead who die in the Lord" (Revelation 14:13).

This verse is not a lesson in horticulture. The casting into the fire and burning are references to the final fate of the wicked who know not God and obey not the gospel of Christ.

And they gather them and cast them into the fire ... Men are not charged with such responsibility as this, the gatherers here being the angels of God (Matthew 13:41,49).

This metaphor breaks down at one point, because branches of a vine have no choice of remaining or not remaining in the vine; but individuals in Christ do have such a choice. This concept is inherent in Jesus' command to "abide in me," the power to do so being implied by the command itself.