To translate this line of thinking from the Middle Ages into the jargon of a modern world, we believe because we first decide to believe. We take a leap of faith. That does not mean there is no evidence that would move us toward belief. There is lots of evidence and there are many circumstances that would move us close to belief, but once we choose to believe, then we begin to see the proof that convinces us of the rightness of what we believe. For this reason, we can feel certain about what we believe.
Truth is self-authenticating.
We believe it because it changes our lives. It feels right and true to us. And once we believe it, then we say, ''Oh, yes, here are all the reasons. It grows in us once we believe, once we come to that place of faith. Otherwise, whatever I found to be incontrovertible evidence someone else could easily dismiss.