From: Matthew 13:24-30
The Parable of the Weeds
 Another parable he (Jesus) put before them, saying, "The kingdom of heaven may be compared to a man who sowed good seed in his field;  but while men were sleeping, his enemy came and sowed weeds among the wheat, and went away.  So when the plants came up and bore grain, then the weeds appeared also.  And the servants of the householder came and said to him, 'Sir, did you not sow good seed in your field? How then has it weeds?'  He said to them, 'An enemy has done this.' The servants said to him, 'Then do you want us to go and gather them?'  But he said, 'No; lest in gathering the weeds you root up the wheat along with them.  Let both grow together until the harvest; and at harvest time I will tell the reapers, Gather the weeds first and bind them in bundles to be burned, but gather the wheat into my barn.'"
24-25. "The situation is clear: the field is fertile and the seed is good; the Lord of the field has scattered the seed at the right moment and with great skill. He even has watchmen to make sure that the field is protected. If, afterwards, there are weeds among the wheat, it is because men have failed to respond, because they--and Christians in particular--have fallen asleep and allowed the enemy to approach" ([St] J. Escriva, "Christ Is Passing By", 123).
25. This weed--cockle--looks very like wheat and can easily be mistaken for it until the ears appear. If it gets ground up with wheat it contaminates the flour and any bread made from that flour causes severe nausea when eaten. In the East personal vengeance sometimes took the form of sowing cockle among an enemy's wheat. Roman law prescribed penalties for this crime.
28. "When the careless servants ask the Lord why weeds have grown in his field, the explanation is obvious: 'inimicus homo hoc fecit: an enemy has done this.' We Christians should have been on guard to make sure that the good things placed in this world by the Creator were developed in the service of truth and good. But we have fallen asleep--a sad thing, that sluggishness of our heart while the enemy and all those who serve him worked incessantly. You can see how the weeds have grown abundantly everywhere" ([St] J. Escriva, "Christ Is Passing By", 123).
29-30. The end of this parable gives a symbolic explanation of why God allows evil to have its way for a time--and for its ultimate extirpation. Evil is to run its course on earth until the end of time; therefore, we should not be scandalized by the presence of evil in the world. It will be obliterated not in this life, but after death; at the Judgment (the harvest) the good will go to Heaven and the bad to Hell.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.