This is a warning to us—that the iniquity that is in the world will cause a loss of love in the church. If we understand the progression of events in Matthew 24, then verse 12 speaks of the time of the Tribulation. We are leading up to that, living in a period in which the stresses against the church—from the world—are increasing. As they increase, it can have the psychological effect—because we begin to get weary of dealing with it—of becoming apathetic, that is, without feeling for what we formerly loved so dearly.
So the iniquity is in the world, but resisting it is a constant stress because it exerts tremendous pressure through an appealing façade—to give in and go along with it. As we live with it and everybody else is doing it, the world's behavior gradually becomes acceptable to us, thus giving evidence that apathy is taking over.
We need to look at every aspect, even in areas we may consider "minor things." How do they dress? What kind of music do they listen to? What are the world's movies like? What are their attitudes in dealing with each other—in stores, on the street, in communities? In many places, we can hardly get anybody on the street to greet us! There are many little behaviors like this. The iniquity is in the world, but it pressures us into doing things as it does—and then it becomes our behavior.
This is just hypothetical, but what if we evaluated ourselves against the world ten years ago and judged that we were 50% more righteous than the world. Then today, we did exactly the same thing, and figured that we are at least 50% more righteous than the world. However, if the world had become more unrighteous during that same period, then, even though we may be 50% more righteous than the world now, we have actually gone backwards in those ten years—right along with the world!