Since we (mankind) tend to be a 'rational' race, we will weigh the evidence before deciding on our belief in something. Evidence can be clear and objective, such as a knife with someone's fingerprints on it. Or, evidence can be subjective and perhaps untrustworthy, such as 'I was told by someone who saw the knife that they knew it was owned by a particular person'.
And, besides evidence, we consider what we DESIRE the outcome or state to be, independent of the evidence.
So, I see 'Faith' as representing our Desire, tempered by rationalizing the evidence. A strong desire with little tangible evidence may lead one to adopt a strong faith in the outcome.
Delusion I see as a desired state that does not have the support of tangible evidence and perhaps even has tangible evidence against the desired state such as similar states that have clearly different outcomes than the one desired. It would be faith in a lie.
In both cases, one needs to reach a clear outcome in order to demonstrate whether or not the belief one has is truthful or a delusion. Sometimes, this cannot be reached in our lifetime. But this does not mean that what we call 'faith' is actually a delusion. A delusion ultimately would be proven to be a lie.