When I choose to believe something, do I thereby consider it to be more certain than I considered it before having chosen to believe?
In favor of this: belief implies holding to something as true, rather than as possibly true or probably true; when I choose to believe something for which the evidence is merely probable, I assent to as true, whereas previously I assented to it as probably true, and so I consider it more certain than I did before believing.
Against this: if surely assenting to something implies that I can treat that as an sure truth in itself, I will necessarily consider my belief to be infallible. Suppose, for example, the evidence strongly suggests that someone is telling the truth. I choose to believe that he is in fact telling the truth. Now I know I hold the belief that he is telling the truth. And I know that in all cases in which I hold a belief that X is true and in which X is true, my belief that X is true is true. So if my belief allows me to take the proposition "X is true" as sure in itself, I will conclude that there is it is absolutely certain that my belief is true. This is manifestly unreasonable. Therefore, giving credence to something does not entail or allow me to treat it as certain in itself. A similar problem will follow if I treat it as more certain than before, except to the extent that my belief is in fact evidence of its being true.
The problem here seems to be that we're comparing apples to oranges in comparing the firmness of assent in believe with my estimation of the certainty of some matter (= subjective certainty established by reasoning). To the extent that belief involves choice, and so an act of will moved by the good, whereas my subjective estimation of an objective probability or certainty does not involve choice (per se) but perception, the subjective certainty arising from will may be analogous, but is not directly comparable to the subjective certainty arising from perception or reasoning.