Narcissism, even within the parameters of the personality disorder, exists on a continuum. Some people are more, some are less. But really, it isn't a straight line scale, it's a landscape of selfishness. Some people cross the border regularly for visits, some people hoist the flag and buy penthouse apartments in the capital city.
By the time I learned about NPD, my mother was very much on the decline physically, so I had to get help from friends who also knew my opinion about her being NPD. My friends are still astonished that I am confident she was genuinely a Christian, they just couldn't find any evidence other than the religious forms she had always used as a cover.
The key thing to remember about Christianity is that the root of it is not in the mind, it is in the person's spirit, and we neither save ourselves nor keep ourselves in a state of righteousness with God by our prayers or good works. Being made right with God is something we give ourselves in trust to Jesus to do FOR us. That is our faith - we are trusting Jesus to reconcile us to God AND keep us from being such stupid jackasses as to walk away from that reconciliation.
Lots of people stop right there. They recognize the truth of the gospel that Jesus is the Way, the Truth, and the Life, they sincerely believe it and acknowledge Jesus as their savior, they never stop believing that is true - and then they carry on the rest of their lives trying to live out of their reasoning faculties. Believing happens in the heart, reasoning comes out of your head.
NPD is an entrenched mental disorder. It begins as a form of self defense and grows over time to become an aggressive, pernicious, hideous thing. I heard someone call mental illness "cancer of the mind" once, and it might be a fair analogy. It makes the personality deformed and ugly, but it's key to remember that "personality" is just the part of a person that we see acting outwardly. The innermost part of a person we never see, only that person and God knows what goes on in there.
It only takes faith the size of a mustard seed for God to change someone completely from the inside out. How many have waited until the last moment to throw down the rebel flag and surrender, yet we have no problem believing they are saved? How many sit in church because it's full of "nice" people, yet never do believe that whole "blood shed at Calvary" rigmarole?
If your Mom says she saved, go with that in your prayers. Hold her words up to God with your whole heart and entrust Him to make her words true and sure. It's His problem and He can bloody well come up with the solution. If He is God, then He can live up to it for your mother. (I'll tell you a big secret about that kind of prayer - He ain't scared of it and He likes to save people right where they are.)
You will have to keep your own head outside of the neat little boxes psychology and religion want to stuff people in. Categories of mental illness and most sermons are ratiocinations of the mind, and faith in God always comes from your spirit. I recommend C.S. Lewis as a trusted ally - The Screwtape Letters, Mere Christianity, The Great Divorce, and the whole of the Narnia books.
Lewis had an understanding of people in the grip of addiction and mental illness, and hard won experience in being a faithful friend to both. I think both of those things light up his writing. "...it is not easy to throw off in half an hour an enchantment which has made one a slave for ten years."
