MATTHEW 13 24 Another parable He put forth to them, saying: “The kingdom of heaven is like a man who sowed good seed in his field; 25 but while men slept, his enemy came and sowed tares among the wheat and went his way. 26 But when the grain had sprouted and produced a crop, then the tares also appeared. 27 So the servants of the owner came and said to him, ‘Sir, did you not sow good seed in your field? How then does it have tares?’ 28 He said to them, ‘An enemy has done this.’ The servants said to him, ‘Do you want us then to go and gather them up?’ 29 But he said, ‘No, lest while you gather up the tares you also uproot the wheat with them. 30 Let both grow together until the harvest, and at the time of harvest I will say to the reapers, “First gather together the tares and bind them in bundles to burn them, but gather the wheat into my barn.”’” 31 Another parable He put forth to them, saying: “The kingdom of heaven is like a mustard seed, which a man took and sowed in his field, 32 which indeed is the least of all the seeds; but when it is grown it is greater than the herbs and becomes a tree, so that the birds of the air come and nest in its branches.”

The Devil is called a man that is an enemy because he has ceased to be God; and in the ninth Psalm it is written of him, Up, Lord, and let not man have the upper hand. Wherefore let not him sleep that is set over the Church, lest through his carelessness the enemy should sow therein tares, that is, the dogmas of the heretics.
(St Jerome)