As Catholics we know that in order to put ourselves into Mortal Sin, we must know the action is offensive to God and Will to do it anyway.  So someone can stop practicing the Faith, even abandon it for another religious practice, or secular life, without really understanding the Faith or the seriousness of what they are doing.  While by doing so they may put their soul in peril, each case is individual, and we cannot judge them, only God who knows the intent of the person can do so. 

What is more important however is to understand the Faith, not just what we believe a Catholics but why we believe what we do.  When one learns not just what the Church teaches, but why it does there is a far greater likelihood that they will remain Catholics.  Many Catholics leave the Church not because of what the Church teaches, but because they don’t know the teachings, and are easily peruaded by others outside of the Church that the Faith is wrong, usually by giving them a misrepresentation of what the Church or the Bible (which the Church is in accord with) really teaches.  Without the ability to know they are being told something untrue, they are esy prey to those who really believe the false accusations against the Church.