Forgive me if I don't like the way you phrased this. You cannot "lose" it like you lose your keys when you try to find them in the morning on your way out the door. You cannot "lose" it like you "lose" a check as it goes through the laundry. You can, through no lack of efficaciousness of the grace of God, but through a lack of a desire or a change of desire to pursue a relationship with God, decide to walk away from a belief and a faith which you once held. Note that I pointed out that this does not happen to people who "mess up." Not even people who "mess up bad" (see King David). This happens to people who attribute the work of God to Satan. This happens to people who LIKE crucifying Christ and shaming Him.
I don't mean to give offense. I have to leave for the day so I would like to take this up with you tomorrow.
Until then, have a good night.
-ksen
800, since God knows whether we will make it or not, would he call someone who he already knows will fail, thereby condemning him to eternal Hell?