Well, that's awfully generic, isn't it? Some people think the spirit of God inhabits nature, wildlife, trees, plants, etc. I wouldn't lose a wink of sleep if they were to lose their faith in that drivel. Or "everyone is a little god". Or "all paths lead to Heaven". Ditto for all the other wacko viewpoints on TOL.
Yes, it does sound heartless. It sounds unChristian.
Let's try an analogy. I don't know your political persuasions, but would you care if someone switched from being against your viewpoints? Probably not. There is a parallel here with Knight not caring about someone dropping a erroneous belief.
He does believe in OSAS, but many Christians don't. For them, a loss of belief in God is by definition, eternally fatal.
Again, losing something that was nothing to begin with--what's the loss?
But Knight relies on the no true Scotsman fallacy by saying that if they lose faith, either they're still saved, or they weren't really a Christian anyway.
The no true Scotsman fallacy does not apply to eternal souls. On this issue there is no wiggle room. You either are a "true Scotsman" or you're not.
His indifference to their fate (if they're saved), in that context, is more understandable. But if they weren't really a Christian by his definition, then his attitude is callous and unChristian.
I think you've got the logic reversed.