I've made the experience myself, and gathered as much from other coming out stories, that many lifetime Christians who haven't encountered other points of view too often have no clear concept of being a good person that is separate from "being a good Christian". It could be that the most important message you should get across to your mom is that even if you can't help but reject the details of the faith she has brought you up with, that does not mean that you've discarded all the values she has probably tried to teach you in that context - family, honesty, you name it. Teaching her that those two can be separate things is not necessarily easy if she's been taught all her life that Christ is the only path to becoming a good person, but probably much easier than convincing her that Jesus is fake and there's no God.
Cheers
Cheers
The fool hath said in his heart, There is a God. They are corrupt, they have done abominable works, there is none that doeth good.
Psalm 14, KJV revised edition
