Actually Christians resort to this argument because it is one of the fundamental beliefs in biblical theology. The bible is absolutely littered with verses that indicate that God chooses us, not vice versa. Therefore, if you were 'chosen' by God, who declares himself to be infinitely faithful, you cannot turn away and 'lose' your faith, it's not yours to give up in the first place.
That's not to say , however, that we're all mindless robots dumbly following pre-programmed orders. I believe God intermingles pre-destination and personal choice in a way we can't comprehend to bring glory to himself (his chief goal).
So in my mind you're either
a) Still a Christian, even if don't consider yourself one (because you now question the validity of the scriptures)
OR
b) Never were 'chosen' and saved, but went through a period of "religiosity" much like the seeds that were strangled by weeds in the parable of the sower (not trying to offend you here btw, please don't take it personally!)
There are however, plenty of Christians who would disagree with me and say you can lose your faith. I have plenty of verses though to back up my -Calvinist- point of view if you're interested.