John Chrysostom — as excerpted in the Catena Aurea on John 15:1-3

“(Hom. lxxvi. 1) And inasmuch as even the best of men require the work of the husbandman, He adds, And every branch that beareth fruit, He purgeth it, that it may bring forth more fruit. He alludes here to the tribulations and trials which were coming upon them, the effect of which would be to purge, and so to strengthen them. By pruning the branches we make the tree shoot out the more.”