Have you ever tried to return something to the store without a receipt? It can sometimes be a hassle, because your receipt is the easiest way to prove you bought that item at that store.
It is your proof of purchase. In verse 8, Jesus tells the disciples that it is by bearing much fruit that they will “prove to be [his] disciples.” They don’t become his disciples by doing a bunch of good things. Jesus makes this abundantly clear a few verses later, when he says, “You did not choose me, but I chose you and appointed you that you should go and bear fruit” (v. 16). Fruit does not make a disciple any more that a receipt makes a purchase. Only Jesus has the ultimate authority to make disciples. But just as your receipt is what proves your purchase, fruit is what proves discipleship. In a perfect world, you’d never make a big purchase without a receipt to accompany that purchase. Even more so, you’ll never find a true disciple that does not produce fruit.