belief is an act of the will
The Catholic doctrine concerning faith and reason is this, that reason proves that Catholicism ought to be believed, and that in that form it comes before the Will, which accepts it or rejects it, as moved by grace or not. Reason does not prove that Catholicism is true, as it proves that mathematical conclusions are true, e.g. ...but it proves that there is a case for it so strong that we see we ought to accept it. There may be difficulties which we cannot answer, but still we see on the whole that grounds are sufficient for conviction. This is not the same thing as conviction. If conviction were unavoidable, we might be said to be forced to believe, as we are forced to mathematical conclusions--but while there is enough evidence for conviction, whether we will be convinced or not, rests with ourselves--This is what the priest means, when he is first asked -If a man has not evidence enough to subdue his reason, what is to make him believe?' and then answers 'His will.' and this is just our trial--and one man rejects what another accepts--On the contrary, were we forced to believe, as we are forced to admit that two sides of a triangle are greater than the third, there would be no trial of our affections, nothing morally right in believing, or wrong in not believing.
(John Henry Newman, in his 12 October 1848 letter to Catherine Ward, quoted here - emphasis and ellipses original)