This passage is entitled DIRE WARNINGS, which certainly makes you sit up and listen.
I think this passage really does follow up with what we have yesterday. We have reference to the children again, but also a reminder that you are either for or against God. There is no third front. You can't choose to not be involved. I think some people think that the default is for God, which this passage doesn't actually deny. It just says who causes little ones to stumble are, well, damned.
Now, I think if you choose to be apathetic you're choosing against God, and certainly the Bible says that at other points. You have to choose forgiveness, choose God, or else the default is against God. And even the "good" rhetoric in our society kind of leans kids against God. We're so busy telling each other not to yuck someone's yum that you can't really evangelize unless someone asks. Maybe it was like that back in Rome as well, but in any case, the idea of the default choice being negative isn't one that our society would like to believe.
Although, if the default was to choose God, I can't imagine our society would be anything like this. But that's beside the point.
The salt thing is a bit more confusing. Salt as a preservative, maybe--through fire, we will be preserved? Salt as bring out flavor--through fire, we will become more flavorful? Stronger? Actually, both of those would be good things. Hmm. Maybe this salt analogy is better than I give credit for. I don't think I'm interpreting it correctly, though.
Also: I should start telling people to "have salt in themself".