There is no one with such a sure mastery of this doctrine [of faith in Jesus] that he need not apply himself to it every hour with great diligence. It may happen that one hour finds you very well acquainted with this doctrine and firmly believing it, while the next may find you weak and not knowing where Christ is to be found; then you are in danger of losing Him. The devil does not take a holiday; and when he comes along with his temptations, this may happen easily. The weakness or the strength of faith becomes apparent when battle breaks; then we discover that faith is not a minor matter.
Luther’s Works, v. 23, 183