What does "if nowhere else" mean here?

"When I write a novel in which the central action is a baptism, I am very well aware that for a majority of my readers, baptism is a meaningless rite, and so in my novel I have to see that this baptism carries enough awe and mystery to jar the reader into some kind of emotional recognition of its significance.To this end I have to bend the whole novel—its language, its structure, its action.I have to make the reader feel in his bones if nowhere else, that something is going on that counts.Distortion in this case is an instrument; exaggeration has a purpose, and the whole structure of the story or novel is made what it is because of belief.This is not the kind of distortion that destroys;it is the kind that reveals, or should reveal." ————Flannery O’Connor

Does it mean at least?But "in his bones" has been an very high degree so I don’t think it makes sense.

"At least" is a close paraphrase. The phrase is a shortened version of the whole. "If" is a conjunction: I have to make the reader feel in his bones if nowhere else that... = I have to make the reader feel in his bones even if the reader feels it nowhere else that...