The typical reply to this is that they were never really a Christian to begin with.
Which is asinine, when you actually take a few moments and think it through.
I mean, if once you are Christian you cannot become a non-Christian, then you do not have free will once you become a Christian....
But then, there are really far to many people who do not think things through.
Although I agree, there are plenty of people who claim to not only know what Jesus would say, but also think that they speak with the same authority they give Jesus.