Sometimes, Christians worry whether they might not have lost their faith. Here is a line of thought that might be comforting, though it is probably only acceptable to Catholics. Faith being a gift of grace, it is not possible to lose faith without losing sanctifying grace. It is not possible, however, to lose sanctifying grace but by committing a (formal) mortal sin. Therefore, faith can only be lost through committing a mortal sin. But in a serious Christian, mortal sin is very unlikely to be something done casually—it is a free and conscious rejection of God's love, after all. Faith is not, then, something one can "just lose". It is something one can reject, but only by a mortal sin.