[T]here is a rather complex [parable] which Jesus explained to the disciples: it is that of the good grain and the weed, which deals with the problem of evil in the world and calls attention to God’s patience (cf. Mt 13:24-30, 36-43). The story takes place in a field where the owner sows grain, but during the night his enemy comes and sows weed.... We all know that the demon is a “sower of weed”, one who always seeks to sow division between individuals, families, nations and peoples. The servants wanted to uproot the weed immediately, but the field owner stopped them, explaining that: “in gathering the weeds you root up the wheat along with them” (Mt 13:29). Because we all know that a weed, when it grows, looks very much like good grain, and there is the risk of confusing them.

The teaching of the parable is twofold. First of all, it tells that the evil in the world comes not from God but from his enemy, the evil one. It is curious that the evil one goes at night to sow weed, in the dark, in confusion; he goes where there is no light to sow weed. This enemy is astute: he sows evil in the middle of good, thus it is impossible for us men to distinctly separate them; but God, in the end, will be able to do so.

--Pope Francis, Angelus address, July 20, 2014