A recent post on Jimmy Akin's blog mentioned the term "motive of credibility" while answering a reader's question about how and when we stop relying on reason alone and make that necessary leap of faith. He explained it as such:
What people who are questioning what they're being told by others need is a reason to believe them, or what is sometimes called a "motive of credibility." The more motives of credibility they can establish regarding the truthfulness of what they are being told, the more reason they have to accept it.
For example, if a person who was blind from birth wants to know why he should believe, on the word of someone else, that grass is green and that the sky is blue, he is asking--in essence--for a motive of credibility. He can't perceive these things for himself, but he's seeking a reason that make the claims credible.
The logical one to offer in that case would be the testimony of others. The sighted person who has just told him that grass is green and that the sky is blue might say, "Don't just take my word for it. Ask other people! They'll tell you the same thing."
While a person blind from birth could never completely rule out the possibility of a society-wide conspiracy of Santa Claus-like scale to deceive blind people about the colors of objects (or even the existence of color itself), each person he talk to who confirms that grass is green and that the sky is blue provides him one more motive of credibility to accept these facts, and at some point the volume of the motives becomes such that (if he is rational), he'll end up saying, "Okay, I can't see these colors for myself, but it's reasonable for me to believe both that color exists and that grass is green and the sky is blue."
I like this term, and the concept is one I've touched on before here when talking about evidence vs. proof, and it's also an important concept in the excellent book, The Science Before Science. When in the realm of things that are simply not provable, it is the motives of credibility that help us to still have reasonable faith that something is true. It is relying on motives of credibility, examining what one faith system says and comparing it to reality as we know it, that helps us acknowledge truth, and helps us to consider one source more reliable than another. (This is how I came to accept Catholicism, at first I checked out every last claim...after finding claim after claim that rang true once I considered it, I started to trust that the Church has the wisdom she claims.)
And that is the difference between having a reasonable faith in something, and just arbitrarily deciding to believe that the latest guru on TV is correct that it is fairies who make the earth go round. This is what atheists and agnostics miss when they compare believing in God to believing in the tooth fairy, in fact most of them would probably say that reasonable faith is quite the oxymoron. But it is they who are being unreasonable in not acknowledging the fact that we all rely on motives of credibility and probabilities of truth every day and for important and vital information, and there is nothing irrational about doing so.