Ludwig Wittgenstein, Culture and Value (Vermischte Bemerkungen), ed von Wright, tr. Winch (University of Chicago Press, 1980), p. 53e:
I believe that one of the things Christianity says is that sound
doctrines are all useless. That you have to change your life. (Or
the direction of your life.)
It says that all wisdom is cold; and that you can no more use it
for setting your life to rights that you can forge iron when it is
cold.
The point is that a sound doctrine need not take hold of you; you
can follow it as you would a doctor's prescription. — But here you
need something to move you and turn you in a new direction. —
(I.e. this is how I understand it.) Once you have been turned
around, you must stay turned around.
Wisdom is passionless. But faith by contrast is what Kierkegaard
calls a passion.
Sound doctrines are useless? It would be truer to say that faith as a mere subjective passion is useless. The fideisms of Kierkegaard and Wittgenstein fall far below the balanced positions of Augustine and Aquinas. The latter thinkers understood that sound doctrine, though insufficient, was an indispensable guide. They neither denigrated reason nor overestimated its reach. Reason without faith may be existentially empty and passionless, but faith without reason is blind and runs the risk of fanaticism.