There is a wide range of interpretations in the last two hundred years about the nature of this fruit. Some say it is “soul-winning,” though there’s nothing here that even remotely implies that. Others say this fruit is obedience or love or Christian character or something similar. But why reduce the fruit only to one of these? The rest of the passage indicates that this fruit is the result of praying in Jesus’ name (v. 7) and the purpose of this fruit is to bring glory to the Father. (vv. 8). So the fruit here is everything that comes as a result of praying effectively in Jesus’ name. Some of the things this might involve are given right here in this text: obedience to Jesus’ commands (v. 10), experiencing Jesus’ joy (v. 11), love for one another (v. 12), and effective witness to the world (vv. 16, 27).
So you will be fruitful in the things that matter for eternity. These are the fruits of a life united with Christ and centered on Christ. Just as the Lord doesn’t give us the same peace that the world gives (14:27), so he doesn’t give us fruit that is of this world but fruit that is of the age to come. This is fruit that matters not just in this life, but that matters eternally. It is not fruit that we could bear on our own, but fruit that we could only bear as a result of abiding in Him.
All of these things come from persevering and enduring in your dependence upon the vine, which is Jesus Christ. This dependence on the vine is driven by faith and affects every aspect of our life and witness. (D. A. Carson, John, 517)
Maximos the Confessor: “…our weakness, when moved to do good things, is unable to bring anything to completion without the giver of good things. The one who has come to understand the weakness of human nature has had experience of the divine power. And such a person who because of divine power has succeeded in some things and is eager to succeed in others never looks down on anyone. For he knows that in the same way that God has helped him and freed him from many passions and hardships, so can he help everyone when he wishes, especially those who are striving for his sake.” (Ancient Christian Commentary on Scripture, IVb: John 11–21, 167).
Here’s one more thing to love about this passage: He doesn’t merely say that you will bear fruit, or maybe you will bear much fruit, he says that if you abide in Christ and Christ abides in you, then you will bear much fruit (v. 5).