Matthew 13:25

Verse 25. Were asleep. When the superiors or pastors of the Church were lulled asleep or negligent, or, when the apostles were dead, as Saint Augustine expounds it, the devil spread the tares or error and sin amongst a great number of Christians. These falling from the state of grace, or becoming heretics, are yet mingled with the rest of the faithful in the same outward profession of Christianity, not unlike the good corn and cockle in the same field.

darnel is a weed grass (probably bearded darnel or lolium temulentum) that looks very much like wheat until it is mature, when the difference becomes very apparent.