If I am waiting, hoping, looking, or longing for some subjective feeling or other to certify my belief in God, I am barking up the wrong tree. Completely. Not to pick on them especially (because they are by no means unique in this respect, but rarely is it institutionalized so explicitly as among them) but the LDS are notorious for this, with their alleged “burning in the bosom” which they claim validates the truth of their religion. The problem is that burning bosoms are a dime a dozen; I produced one in myself just now while typing this. Does that invalidate the LDS claim? Obviously not, by itself: it is subjective. But it does highlight the perils of relying upon such things to validate the truth.
The same reliance can be found whenever I say to myself that it feels as though my prayers are bouncing off the ceiling, or when I hope for a sign to give me confidence about what I am supposed to do or think or believe. The problem is that God can’t be touched, can’t be felt, can’t be seen (John 1:18; 1 John 4:12). He may be heard, but He has spoken for the last time in His Son (Hebrews 1)…and even if He did speak, how would I know it is He and not Satan dressed up as an angel of light (2 Corinthians 11:14)?
Faith is in a real way a sort of darkness, which is why St. Paul says that we don’t walk by sight (2 Corinthians 5:7). So when the way is blurry, or fogged, or just plain impossible to see with my eyes, with my senses, I can still know the way by faith. It is an uncomfortable thing, it is a sometimes painful thing, it often goes on longer than I think I can bear…but it is the only way I can go. By faith.
Aquinas, Etc. 
Considering that salvation is often talked about as light in the Bible, it feels weird to call faith a sort of darkness… but I agree. Through faith in God we know our destination is Heaven. Through faith in God we know how to walk in His will. But the path ahead of us is unknown. We don’t know what will happen tomorrow, or the next day (Eccl. 8:7, 9:12, 10:14). We don’t know when Christ will return (Matt. 24:36). Faith leaves us ignorant of what will happen next, trusting God to know best, and accepting whatever He does.
Faith is a sort of darkness, when it comes to the future. Faith is about not knowing what will come next, but keeping in mind the ultimate destination and following God regardless of what ends up happening (John 20:29).
I think we are on pretty much the same page. I would not want (and certainly didn’t intend) to deny that faith is a light in important ways. But precisely because faith cannot be felt, seen, heard, or otherwise sensed there is a way in which it is beyond our powers, and that is the darkness I have in mind in the post.
Thanks for commenting!
Fred
I didn’t figure you intended to deny that, don’t worry. :)