This Sunday's Gospel is about the weeds growing among the wheat. One commentary I looked at said that the Greek word used for "weeds" refers to a plant that looks a lot like wheat, especially when young. It puts an interesting twist on the parable, because it makes it harder to tell the good plants from the bad.
You could say a lot about this parable, so here's just one thought that came to mind. It reminds me that life isn't perfect, and the people I live with aren't perfect, and I'm not perfect either. We all have to put up with a lot of inconvenience and hassle from other people. The weeds are out there growing in the church and everywhere else. That doesn't mean we should be complacent about evil (especially when it comes to very serious evils) but we shouldn't be surprised by it.
I think Jesus was telling us to be realistic and expect that we'll have to deal with a lot of evil in life. Don't expect the Church to be a group of saints.