The reason has to do with Matthew's symmetrical structuring of the entire parables discourse (ch. 13). In verses 1-23 he also began and ended the segment with the telling and the explaining of the parable of the sower, bracketing a section in the middle (verses 10-17) highlighting the purpose of Jesus' parables: selective communication to true disciples.
Here too the telling of the parable (verses 24-30) and its explanation (verses 36-43) serve to bracket the intervening material, so as to form an interpretive framework for it. The parable of the wheat and weeds is about the spiritual conditions that will prevail between the time Jesus spoke this parable and his coming again to set up the kingdom of God. At that time the wheat (believers) and weeds (unbelievers) will be separated and judgment will be passed. This is what John the Baptizer described as Jesus' task in inaugurating his kingdom. But what John could not see at the time was that there would be a long interval of time between Jesus' first coming to die and rise again, and his second coming to judge and set up his universal kingdom on earth. In the meantime the form that God's kingdom will take is described by the intervening parables of the mustard seed and the yeast. So first I am going to focus on the frame—the parable and its explanation by Jesus—and then on the intervening description.
Responsible scholars of the gospels are coming to the conclusion that in order to understand Matthew's (or Luke's) interpretation of the Lord's parables, we must pay careful attention to his structure, sequence and context. A good explanation of this interpretive approach can be found in Charles H. Talbert's Matthew (Grand Rapids: Baker, 2010), pp. 162-65.
Both Matthew's original readers and today's readers are also expected to react to the stories and allow them to adjust our thinking and challenge our attitudes and current behavior. A reader's initial reaction may differ from his final one after hearing the entire parable and reflecting upon it. For example, the initial reaction to the sower parable might be shock or even outrage at the apparent waste of the seeds that fell in the wrong places. The initial reaction to the parable of the wheat and weeds might also be horror at the thought that the "enemy" has ruined the field owner's field. but as the story unfolds, readers see that the weeds do not choke out the wheat (as the thorns in the parable of the sower), but merely co-exist. The reader's attitude changes then from horror to curiosity: what is the story telling us about why we should not fear the mixture?
In this parable the first sower sows "good seed". In the first parable all of the seed as good, but it isn't called "good" because no "bad" seed will later be mixed in with it by another sower. In the parable of the sower, some of the locations where the seed fell were good and others not so good. The adjective "good" is necessary in this case, because a second sower will come at night and secretly try to sabotage his crop by sowing bad seed—that is, "weeds"—in his field. This second person is described as "an enemy." We aren't told why the enemy was sowing the weeds. Did he hope to ruin the first man's crop? The weeds do not crowd out or reduce the growth of the wheat. It may be that he thought the field owner would not know how to handle the resulting mixture, and the wheat would become unusable.
The field-owner's servants were perplexed when they saw the weeds appear. At first they wondered if their master had used inferior seed that already had a mixture of weed seeds in it. But the field owner knows what has actually happened and tells them that an enemy has done this. The servants offer to go into the fields and try to uproot the weeds in their immature state. But the field owner knows that there is a danger that some wheat will be mistaken for weeds and uprooted. This he does not wish to happen. When the wheat is fully mature at harvest time, it will be much more easily distinguished from the weeds. Then it will be easier to separate the two. The weeds will be gathered first and burned. Then the wheat will be harvested and stored in his barn.
How is this a picture of the kingdom of Heaven? Jesus gives his explanation after he has sandwiched in two more parables which we will discuss tomorrow. Those parables elaborate on the peculiar conditions that will characterize the period in the kingdom of God when the wheat and the weeds are growing together in the same field and their separation at harvest time has not yet come.
This is how Jesus explained the parable. Notice first in verse 36 that Jesus only explained the parable privately to the inner circle of his disciples, not publicly in front of the crowds, and that he gives the explanation only after his disciples ask him for one.
In the first parable the sower could be Jesus in his Galilean preaching, but it could also depict his disciples in both their present and future preaching. In this parable the sower is Jesus himself. Whereas in the first parable the seed was the message of the kingdom, the word of God, in this one the good seed are believers whom Jesus "sows" in the world. Jesus says clearly that "the field is the world." In the Sermon on the Mount Jesus called his disciples the light of the world (Matt 5:14-16) and the salt of the earth (5:13). Here they are the good seed, put in the world to grow mature and show the edible product of their good works. The enemy of the Sower is Satan, the "evil one," who sows unbelievers and opponents of the gospel of the kingdom in the same field, which is the world. The harvest is the end of the world, and the reapers are the angels of God. At the end of the world, God's angels will gather out of the living and the dead those who do not believe, and they will be taken off to punishment, symbolized by the burning of the weeds. The believers will be left in the earth, where God's kingdom will now take its final form, in the new heavens and new earth (Isa 65:17; 66:22; 2 Peter 3:13; Rev 21:1).
This is where the function of the parable that we mentioned above becomes important. The story asks hearers this question and invites us to ponder it. For Jesus (or Matthew) to give us the answer would destroy that purpose. God wants you and me to think about that question and formulate our own answers. If I tell you mine, you might give up thinking about it yourself, thinking that "the teacher is better able to answer that than I am."
Now how about the material that Matthew sandwiched between the telling of the parable and Jesus' interpretation of it?
31 He put before them another parable: “The kingdom of heaven is like a mustard seed that someone took and sowed in his field; 32 it is the smallest of all the seeds, but when it has grown it is the greatest of shrubs and becomes a tree, so that the birds of the air come and make nests in its branches.” 33 He told them another parable: “The kingdom of heaven is like yeast that a woman took and mixed in with three measures of flour until all of it was leavened.” (Matthew 13:31–33 NRSV)
Nor should we try to read anything into the fact that in the first story the actor is a man and in the second a woman. The gender of these is simply determined by their activities: planting was usually a man's work, while making bread a woman's.
What the two stories share is that something that starts very small quickly grows to something big that fills everything.
But even today, after twenty centuries of evangelization and the spread of Christianity in the world, it is by no means filling the earth. A very large portion of the earth's population is still non-Christian, and not just "neutrally" so, but actively promoting alternate faiths. Should we despair? Should we listen to the non-Christian voices in the world who insist that we are entering a "Post-Christian World," and that within a matter of another century Christianity will virtually cease to exist? These parables are the Lord's word to us that his gospel will triumph in the end, not because we are so successful, but because he is the Lord of history and of Creation, and his purposes will never fail.
34 Jesus told the crowds all these things in parables; without a parable he told them nothing. 35 This was to fulfill what had been spoken through the prophet: “I will open my mouth to speak in parables; I will proclaim what has been hidden from the foundation of the world.” (Matthew 13:34–35 NRSV)
Earlier in this chapter (13:10-17) his disciples asked Jesus why he always spoke to "them" (that is, mixed crowds containing hostile critics) in parables, and Jesus himself gave his answer: It was to allow selective communication to open minded and sincere hearers, while masking the meaning from the hostile. Here an additional reason is given, this time not given by Jesus himself but by Matthew under the inspiration of the Holy Spirit. It was to fulfill a passage of Old Testament scripture—Psa 78:2—which Matthew quotes in verse 35. It is worth our time to read together the context of that verse in Psalm 78, for it is informative in view of the context of Jesus' ministry in Galilee as Matthew has described it.
1 Give ear, O my people, to my teaching; incline your ears to the words of my mouth. 2 I will open my mouth in a parable; I will utter dark sayings from of old, 3 things that we have heard and known, that our ancestors have told us. 4 We will not hide them from their children; we will tell to the coming generation the glorious deeds of the LORD, and his might, and the wonders that he has done. 5 He established a decree in Jacob, and appointed a law in Israel, which he commanded our ancestors to teach to their children; 6 that the next generation might know them, the children yet unborn, and rise up and tell them to their children, 7 so that they should set their hope in God, and not forget the works of God, but keep his commandments; 8 and that they should not be like their ancestors, a stubborn and rebellious generation, a generation whose heart was not steadfast, whose spirit was not faithful to God. (Psalms 78:1–8 NRSV)
Check back on Thursday the 29th for the next segment of Matthew 13.