Faith is a particularly slippery term to the partially insane, because it can be used to justify illusions as well as truth. If we believe an illusion strongly enough, it will become true to us (even if we have to dip into psychosis to prove its truth). Conversely, if one lacks faith in any truths beyond one’s self, the world becomes a very nihilistic place in which to dwell.
So when do we know that faith is warranted? There are no easy answers. But you can be sure that if one has to invert a whole frame of reference to justify a belief, that belief is probably wrong. The Course is practical, as ever. And I know from experience that, given a strong will, one is frequently tempted to invert a frame of reference in an attempt to preserve an egoistic dream.