I do not seek to understand that I may believe, but I believe in order to understand. For this I believe–that unless I believe, I should not understand.
That is quote number #41 in the list of 100 Catholic quotes at Sancta Sanctis. Bonus marks if you were able to guess it is from St. Anselm.
I also like this one, number #57, from St. Aquinas:
Believing is an act of the intellect assenting to the divine truth by command of the will moved by God through grace.
There are many more… the list was seven hours in the making.