Beauty is that which, being seen, pleases; when someone encounters the beautiful, he desires to rest in it. A novel about resting in beauty is unlikely to be a great novel; it may be very poetic, but it probably won't be very interesting. Novels tell stories, and stories are about conflicts, and where there is no conflict -- and only the perverse are conflicted about resting in beauty -- there is no story.
So yes, the modern evidence is that great novelists are not greatly devout; even the great Catholic novelists have not, as a class, been marked by their sanctity. But I think it's wrong to interpret this evidence, as some do, as meaning that Catholicism is somehow opposed to great novels, much less to great art. Rather, I think that doubt strengthens a desire to novelize, while trust weakens it. (Provisionally, I'd say doubt and trust work the other way round on the desire to versify.)
Obivously a novel has to have conflict but that surely doesn't preclude non-doubters from writing beautifully of conflict, does it? The greatest conflict of all time is the spiritual one between good and evil and to describe that I'm not sure why being a doubter 'helps'. (As a unrelated aside, I'm interested in the connection between doubt and sanctity, in that there is more merit in 'not seeing and still believing'. When I read recently that Mother Teresa was racked by doubts at times.) Bernanos, in "Diary of a Country Priest" understands the great spiritual battles hidden in the ennui of our lives and and that is why some call it the most Catholic of novels. Ralph McInerney said recently that in this novel Bernanos, who was fiercely conservative (to the point of being a monarchist), goes where many other Catholic novelists (including Mauritain and Powers) fear to tread.