As far as I’m concerned, what she presented as “the only reason to believe” isn’t even a reason at all. Firstly, it’s nothing more than a threat: do this, or you’ll be punished. But secondly, the nature of the threat is such that the only way to take it seriously is if you already believe anyway.
Let me see if I can make an illustration. Suppose I were to tell you that Frodo Baggins saved us all from eternal enslavement by destroying the One Ring. Furthermore, I tell you that if you don’t believe me, the dark wizard Sauron would cart you off to Mordor to torture you for the rest of your life. Would you find that a compelling argument to believe that there’s a real-life Frodo Baggins who saved us from real-life enslavement by destroying a real-life One Ring? Barring serious issues separating reality from fantasy, I’m pretty sure you wouldn’t give that argument a moment’s consideration for one simple reason: you probably don’t believe Sauron or Mordor are real. Therefore, the threat isn’t real. It can’t possibly frighten you.
But even so, what if I were to tell you that if you didn’t believe that Frodo Baggins saved us all from eternal enslavement by destroying the One Ring, I would imprison you and torture you. That seems a little more of a threat, right? I exist, after all, and you can probably satisfy yourself of this fact to a reasonable degree of certainty. So maybe you would tell me that you believe in Frodonic salvation, for the sake of avoiding torture. But you wouldn’t really believe it, now, would you? You wouldn’t be actually convinced of the existence of a humble Hobbit and his ring of invisibility. Because the threat has absolutely nothing to do with demonstrating the truth of the claim.
Oh, also, I imagine you’d probably consider it pretty immoral of me to make such a threat to begin with.
To bring it a little closer to home: suppose I were to tell you (assuming you’re religious) that if you didn’t stop believing in your god, you would be tortured or killed. Not only would you probably not find that a compelling reason to change your actual belief (even if it compels you to tell me you did), but you’d think I’m an asshole (if not outright evil) for making the threat. Guess what: it’s no different when you do it. And it’s no different when the writer of a “holy book” does it, either.
So what it comes down to is this: if you ever find yourself on the point of saying that the only reason to believe in your god is hell, or eternal damnation, or anything similar, you’re really at the point of admitting that there’s no reason whatsoever.