If you were raised in the faith, if religion was part of your upbringing, if Jesus is the center of your mother's world; don't tell that you no longer believe, don't mention that you're agnostic or atheist, don't break your mother's heart. It wouldn't hurt you to go to church with her on Sunday(or Saturday) when she's around, you haven't lost your voice, just saving her from heartbreak. You are not pretending, you're choosing kindness. You are not any less agnostic, it doesn't change your "new" belief. No one, not even your mother can take away your choice, ultimately, you choose. Spare her sleepless nights, praying for your "salvation", you know she'll be at those prayer meetings, standing up, sowing seed, bending God's ear, fasting. If your mother is anything like mine, do her a favour, don't break her heart.