Believing the wrong thing in any area of life can be disastrous especially if it concerns eternal truth. In this passage, John tells us to test the spirits.
The culture of Christianity has been transformative to Western civilization and to our own lives. This is largely because of what John says in this paragraph.
We are still waiting for the coming antichrist, but there are already many antichrists in the world. In this passage, John tells us how to recognize them.