Do people gather grapes from thorns, or figs from thistles? Every tree is known by its fruit (Mt. 7:16)
How can you speak good things since you are bad? Here is another comparison for you to consider. Don’t gather grapes from thorns; after all it’s impossible for grapes to be produced from thorns. But haven’t you noticed a vine shoot getting into a hedge as it grows, and entwining itself in the thorns, and putting forth buds among the thorns, and producing bunches of grapes? You are hungry, and you are walking past, and you see a bunch hanging amid the thorns, you don’t pick anything, you don’t pick it. You are hungry and you want to pick it. Pick it then; stretch out your hand carefully and attentively; beware of the thorns, pick the fruit. In the same way too, when a bad or worthless man speaks the teaching of Christ to you, listen to it, accept it, and don’t ignore it. If he is a bad man, the thorns are his; if he says good things, it is that bunch hanging among the thorns, it isn’t growing from the thorns. So, if you’re hungry, pick it, but watch out for the thorns.
You see, if you begin to imitate his deeds while gladly listening to him, you have been careless in stretching out your hand; you’ve encountered the thorns before reaching the fruit. You emerge wounded, you emerge scratched and torn. The fruit coming from the grape is now no use to you, but the thorns, springing from their own proper root, are doing you damage. Notice, I mean, to avoid being misled, where you have picked the fruit from; it’s a vine branch. Keep your eyes on the branch, and see how it belongs to the vine, comes from the vine, proceeds from the vine, but encounters the thorns. So the vine doesn’t have to draw back its branches, does it? In the same way too the doctrine of Christ, as it has grown and made progress, has entwined itself with good trees, and entwined itself with bad thorns. It is thus spoken by the good and the bad. It is for you to notice where the fruit comes from, where what feeds you comes from, and where what pricks you springs from. They are mixed together in public appearance, but distinct from each other in their roots.
Saint Augustine of Hippo, from Sermon 340A