Perseverance and leaving the fold

On a previous post about the "no ex-atheist"position mattghg wrote:

Surely, though, the "no ex-Christian atheists" view is mandated by the P of a Calvinist's TULIP?

I think this is an empirical problem for point 5 to be honest with you. Point 5 advocates use this passage to buttress their position, I John 2: 19

"They went out from us but they were not of us; for if they had been of us, they would no doubt have continued with us; but they went out, that they might be made manifest that they were not all of us."

But can that be generalized to all instances of "leaving the fold?" Doesn't this commit you to an a priori theory of people who leave, which isn't supported by the evidence?