Boswell actually says, by my reading, that Johnson’s Rasselas will fail to convert anyone who does not already believe in fallen nature and the Christian afterlife, whereas it will proffer wisdom and enjoyment to those who do. This seems to me a qualified praise of the book, since presumably one might produce a work so compelling that it could change the mind of a dissenter. Or maybe this hypothetical work simply serves a different purpose, and Rasselas is intentionally a kind of sermon to the faithful, unconcerned with apologetics.
It could also be that Boswell’s comment is aimed at human nature rather than Johnson’s book. Those who disagree with the assumptions of the book will likely be hardened against its message and will go on dreaming as they did, whereas Johnson will convert again those who are already convinced.