I have had several ongoing conversations with some friends about whether or not people who are really saved can sin and whether people who sin are really saved. So here is the analogy God has shown me that I think sheds light on what I believe this passage answers these questions.
If I invited you to my house for homemade chicken and noodles, you would have a choice to make. To go or not to go. But what if you had never been there before? Since we were together, I wouldn't give you directions, I would ask you to follow me because I know how to get there. You would then follow me by pointing your car in the same direction as mine and driving the same direction and speed as me. You'd stop when I stopped and turn when I turned. You'd get to my house because you stayed on the tail of someone who knew the way.
Of course this applies to our decision to follow Christ. Sadly, I think people often make a mental assent to the idea of what Christ did for us but never really follow. And some get in their cars but never turn and face the right way. They never repent of their sins. And still more repent and begin to follow, but refuse to follow hard enough to stay up. They don't turn where Jesus turns or stop where He stops. They leave the path and go their own way. In the analogy then, they never arrive to the right destination unless they go the distance. I think that applies to salvation.
I think if you read the Bible as if you had never heard anything about it, I might come to the conclusion that salvation was a life-long process not a one-time decision. I get saved from stuff every day. Of course I made the decision at one point, and I don't have to make the decision over and over, but I have to continue to follow. John seems to be saying here what Jesus said Himself, "If you continue..."
This also applies to the sin thing. Of course people following Christ might hesitate at certain intersections and fall behind at a stop sign here and there, but as long as you continue on your path behind the lead car, you'll end up in the right place. It's when you continue in sin like verse 18 talks about, that you are not following Christ. You can't follow your own selfish sinful flesh and not get lost. You can't be a "fully devoted follower of Christ" if you won't stop following something else.
Well, this is pretty long, but I guess it's a pretty serious topic. I wonder what any of you think about it.