Therefore it is given for a daily pasture and sustenance, that faith may refresh and strengthen itself so as not to fall back in such a battle, but become ever stronger and stronger. For the new life must be so regulated that it continually increase and progress; but it must suffer much opposition. For the devil is such a furious enemy that when he sees that we oppose him and attack the old man, and that he cannot topple us over by force, he prowls and moves about on all sides, tries all devices, and does not desist, until he finally wearies us, so that we either renounce our faith or yield hands and feet and become listless or impatient. Now to this end the consolation is here given when the heart feels that the burden is becoming too heavy, that it may here obtain new power and refreshment.
-- Luther's Large Catechism on the Sacrament of the Altar