There is in all human beings a natural desire for beatitude, for a happiness so complete that the desire for it could not be satisfied by the contemplation of any God which reason alone could know, but only by the vision of God of a directness and immediacy which reason is absolutely powerless to achieve and of which it cannot even know the possibility. Therefore, what human beings naturally desire cannot be satisfied by what human beings can naturally know. If follows from this…that even that natural desire for God, which must be frustrated by the incompleteness of the contemplation of any naturally known God, cannot be known in its full character of frustration, except from the standpoint of faith. For it is only by faith that we can know of the possibility of that complete vision of God to which human reason fails to attain. Hence, the ‘noble genius’ of the pagan philosophers – of Plato, Aristotle, Plotinus and Proclus – who did know God by reason, and who, as Thomas says, could experience only a ‘great anguish’ of frustration at reason’s limitedness, did not know the true nature even of their anguish, for they did not, and could not, know that goal of human desire and knowledge by the standard of which theirs fell short.
(Denys Turner, Faith, Reason and the Existence of God)