People think: Doing good works is a heavy task, but believing is
something that is soon done. To be sure, faith does seem to be an easy
matter; but it really is a difficult art. Temptation and experience
certainly teach that, on the contrary, we must say that clinging to
God's Word so that the heart is not afraid of sins and death but trusts
and believes God, is a far more bitter and difficult task than observing
all the rules of the Carthusian and monastic orders.
Martin Luther, cited in "Luther and the Holy Spirit: why pneumatology still matters" Currents in Theology and Mission , April, 2007 by Jeffrey K. Mann