Lets see.. Well I find it significant that in the beginning of Matthew 7 it talks about not judging others, although to me that is not just a negative statement. There was another thread where those first few verses were discussed, but the point I'm making here is that judging requires a subjective mental framework, a filter through which you see things. So we can see how this could be the case with those that thought they did all these great things in Jesus name, they may have really believed in what they were doing, but apparently belief is not where it's at.
Without really going into the subject of viewing what Jesus was saying literally or personally, it's probably helpful to note that in the first case it was the 'workers of lawlessness' who thought they were doing things in Jesus name, in the second case it is Jesus who was making that statement. To me 'knowing' Jesus in the personal sense is irrelevant, I know people who are basically on the ball spiritually and I don't need to know anything about their personal life, those are just details that aren't particularly relevant. So to me it isn't even a question of denomination, different religions (though it helps not to take religion too seriously) is not really important either, in fact in light of the first paragraph I would say it is more likely for a person's belief structure to hinder them spiritually, whether they call themselves Christians is not particularly relevant as the scripture points out.
Incidentally, in terms of casting out demons I would say it's a matter of seeing through them - that they are not real. If there is a factor of clear seeing then that has to be what it means, then again in my world egos aren't real either, but that's another story.. Basically I am talking about what is ultimately real, both of these things are only relatively real.