Every branch in me that beareth not fruit he taketh away: and every [branch] that beareth fruit, he purgeth it, that it may bring forth more fruit.
Note 47 at Joh 15:2: What is this fruit that Jesus was speaking of? Some have taken a very narrow interpretation to say that it is producing another Christian, and therefore any believers who don't reproduce their faith will be taken away. That cannot be verified by other scriptures.
This fruit could be referring to many things. Paul spoke of the fruit of the Spirit (Ga 5:22-23), the fruits of righteousness (Php 1:11), new converts as fruit (Ro 1:13), and holiness as a fruit (Ro 6:22). Jesus could have been speaking of any one or any combination of these things. It is most probable that He was simply using the word "fruit" to denote any of the virtues that God's Word advocates.
Note 48 at Joh 15:2: This purging has been interpreted in many ways. The illustration that Jesus was using is one of pruning; therefore, some have said this purging is a very painful process where the Lord cuts and slashes us through things like sickness, death, poverty, and other forms of tragedy so that eventually we will bear more fruit. This teaching promotes problems not only as a good thing but also as a necessity if we want to bear more fruit.
That thinking is not consistent with the rest of God's Word or even the context of this verse. The Greek word used for "purgeth" in this verse is "KATHAIRO," and it means "to cleanse" (Strong's Concordance). In Joh 15:3, the Greek word translated "clean" is "KATHAROS," which is akin to KATHAIRO. KATHAROS means to "free from impure admixture" (Vine's Expository Dictionary). The text makes it very clear that the purging Jesus spoke of is done through the word that He has spoken unto us (Joh 15:3).
Paul said in 2Ti 3:16-17 that God's Word was given to us for "doctrine, for reproof, for correction, for instruction in righteousness: That the man of God may be perfect, throughly furnished unto all good works." That is God's method of pruning us, and He doesn't need the devil's help. His Word will make us "perfect, throughly furnished unto all good works."
That is not to say that we cannot learn through tragedy. We can. But God has a better way. If we mistakenly think that God is bringing tragedy into our lives to make us more fruitful, then we'll not resist the tragedies, and they will not flee from us (Jas 4:7). All of us will learn by hard knocks, but those who welcome that with their arms open wide will suffer greatly and be far behind those who let God's Word have its perfect work in them.