C4K, the word 'fallen' there is, in the Greek, ekpipto, which means 'to drop away', 'to be driven out of one's course', 'to become ineffecient.'
The NIV also translates this verb as 'fallen'. It is only used ten times in the New Testament and what I had to smile about was that two of the times the NIV translates it as 'run aground.'
One of the things that occurred to me as I was looking this up was that it might not mean 'losing salvation', which may be how some take it. Rather, I'm thinking of the sheep straying and how Jesus will go after it. We do have a tendency to want to depend on laws of one kind or another, rather than grace, and the two are antithetical. So when we prefer any law (not encouraging illegal activities here, folks!) to the grace of Christ where our spiritual state is concerned, then we have wandered away from grace, or perhaps considered it ineffectual at that point. If the person is born again I have no doubt that Christ will go after that 'ship that has run aground' and haul it back into the deep, safe waters of His grace.
Mixed metaphor and all that, but that is where my thinking is leading me at this moment.