Summary: The role of branches is to ebar fruit. The key to bearing fruit is to remain firmly attached to the vine, relying on the power of God to bring forth fruit.
What does it mean for you to be a Christian? Does it mean, perhaps, the comfort of knowing that your on God’s side? Does it mean being part of a wider community of people who are mutually dependent and supportive? Does it mean being part of a world-wide community of people who share a love for and who worship the one God? Does it mean having roots and connections that you wouldn’t otherwise have? Well, being a Christian might mean all of those things to different people, but none of them is at the heart of what Jesus says it means to be Christian. Here in John 15 (quickview)  Jesus begins to teach his disciples what it means to be one of his followers, now that he is leaving them.
He says the whole point of being one of his followers is to bear fruit to the Father’s glory. In fact, he says if you don’t bear fruit, there’s something wrong. Have a look at John 15:1 (quickview) . He begins with the statement that he is the true vine and his Father the vinegrower. Now for the disciples this would have rung loud bells. The image of a vine or a vineyard was a common image of the nation of Israel. That’s what we read in our first reading from Is 5. There, in the song of the vineyard, God tells how he planted a vineyard, meaning the people of Israel, and looked for it to bear grapes, but all he found were wild grapes, of no use to him. And so he says he’ll tear down that vineyard, make it a waste, because it didn’t bear the fruit that God required from it. Israel had failed to do what God required, and so would be left a desolate wasteland. But now Jesus has come and he declares that he is the true vine. He is the one who will obey the Father and please him in all he does (v10) the way Israel failed to do.
But if Jesus is the vine, those who have joined him as his followers are grafted in and become branches of that same vine. And that means that we too are expected to bear fruit. If we don’t bear fruit, the Father, the vine grower, will prune us. He’ll take away those parts of us that are unfruitful, or that are bearing wild grapes, and those parts that are bearing fruit he’ll prune to make them even more fruitful.
Have you ever observed how a grape vine grows? It first sends out shoots, some of which have flowers on them which become grapes. But then about the time that the grapes are beginning to ripen, it’ll suddenly have a spurt of growth. New branches spring out of the vine, new growth. To the casual observer it looks wonderful, full of health and vigour. But the vine grower isn’t interested in lush growth, he’s interested in luscious grapes. So one of the jobs of the vine grower is to go along and prune those branches that don’t have any fruit on them. In fact I was speaking to some grape growers in January and they were saying how in a good year, they’ll even go along and cut off some of the bunches of grapes so that those that are left are even better.
So here’s the first thing we learn from this picture of Christ as the vine and us as the branches. The whole point of being a follower of Christ is so that we can continue his work, to bear much fruit to the glory of God.