The passage in question is part of the parable of the wheat and the tares. I grew up thinking that it was about the church, specifically, that it was about the mixed nature of the church. I find out today that many scholars have thought this throughout history, although they have been somewhat less than thorough about the meaning of this particular verse.
In the Federal Vision controversy, much has been made of the fact that the field is the world, and not the church. The implications drawn by FV proponents from this fact went far beyond the text, however, as they made claims regarding the non-mixed character of the church, the “All or nothing,” as Klaas Schilder would say.
The position I now hold is that the verse talks about wherever Christians (wheat) are found, including the church. There are two main exegetical reasons for this. The first is verse 24, which tells us that this parable is about the kingdom of God. Now, the kingdom of God can refer to God’s overall reign. However, it can certainly refer to the church as well. At any rate, the kingdom of God includes the church. What really ices the argument, however, is verse 25′s phrasing “among the wheat” (ἀνὰ μέσον τοῦ σίτου). This plainly indicates that the weeds (probably a form of darnel, which looks a lot like wheat while growing up) are sown wherever the wheat is. So, by the FV’s own argument, this parable has to be talking about the church, since the church is where all the wheat is. Of course, most would admit that there are Christians outside the church. However, almost all Christians belong to the church. Therefore, the parable is mostly about the church in its mixed character.