This all sounds good to me (apart from the idea that without religion we have no reason to think any life worth living and with it we do have such reason) so long as: a) such a religious text really is both necessary and available (Fleischacker argues that the Torah is such a text, his reviewer has doubts), and b) "positing a metaphysical ontology" doesn't mean what I think it means.
A genuinely compelling attitude should (surely) be one that we are compelled to take, that we find ourselves in, whether we like it or not. And we should, in some sense, like it. That is, we should not think it would be better if we had some other attitude. For instance, I might feel guilty about something, and not enjoy this feeling, but if an attitude of guilt toward my past deeds is compelling then I won't believe that it would be better if I didn't care what I had done. (I'm thinking of Wittgenstein's example from the Lecture on Ethics, but also the novel Sorry by Zoran Drvenkar, which takes guilt and its avoidance as the theme for a murder mystery.) Perhaps it is true that "the natural question to pose would be: What would have to be the case for this attitude to be correct, that is, for it to be appropriate to reality?" But I have to ask whether it is natural to pose any question at all in a case like this. Why not accept the guilt, or the wonder, or whatever it might be?
One answer might be a felt need to find a way to express it. If this need is genuine then the attitude will be expressed, one way or another. One way might be through language, and this language might turn out to be religious (in the obvious sense of including words like 'God'). And then this religious language might be more or less important to your life. You might only shout "God!" or "Damn!" when you drop something or get hurt, or you might find yourself caught up in much more complicated forms of language, in much more seriously or fully religious ways of thinking, talking, and behaving. In the latter case I imagine you would be religious. But that doesn't mean that you will be positing a metaphysical ontology in any straightforward sense. Faith is not positing. Or so it seems to me. The attitude involved in positing an ontology in order to make sense of some experience sounds like a scientific or philosophical attitude, not a religious one.
This all strikes me as being related to some things Robert Gay says, which I wanted to blog about (or at least quote) anyway. Here is Gay on "Bernard Williams on Practical Necessity" (Mind Vol. 98, 392, October 1989, pp. 551-569):
[S]uppose that someone, seeing another person dying, has a sense that human life must be respected, and tries to give voice to this by saying that human life is sacred. We surely should not suppose that he is expressing a conscientious 'sense of duty', ready to summon up efforts of will. Rather, the person's mind may be entirely occupied by his imaginative echoing of the person's fear and his struggles to live, and by his own regret at what is happening. And, if he is entirely possessed by these, there will be no gap for the motivation of effort to fill, nor even the idea of a gap for the motivation of effort to be 'in the offing'.
We may be reminded of Ryle's argument that the enjoyment of, say, a game of tennis cannot be the experiencing of sensations of pleasure, because the more one is enjoying the game, the more completely one's attention is occupied by it, and sensations of pleasure would have to enter one's awareness and distract one from the game. In the same way, this sense that human life must be respected cannot involve the 'sense of duty', because this sense of the sacredness of life involves being entirely possessed by certain thoughts, and if one's mind is filled by those thoughts there is no room for the 'sense of duty', which only occurs to supply deficiencies. If we may picture the 'sense of duty' as black, and the other feelings and thoughts as, say, red, then we should colour the whole of the subject's consciousness at that time red, without any black patches or even a black nimbus around the edges of the red. The 'must' of practical necessity will have to be expressing nothing distinct, but rather (if I may pursue the metaphor one step beyond my explanation of it) the compelling brightness of this red.