To say that faith is required for salvation is not to say that a person can produce faith himself. Scripture requires everything of a person. Every commandment is a demand, crying: “Do this and you will live.” Scripture demands that we “purify (our) hearts.” We are told: “Awake, O sleeper, and arise from the dead, and Christ will shine on you.” But just because these demands are given does not prove that a person can meet them. There is an old trustworthy saying: “Based on an obligation, one cannot be certain of its fulfillment.”
When a creditor demands payment, this does not prove that the debtor can pay. In everyday life, a creditor who knows that his debtor is insolvent may demand payment of a debt merely because he has observed that the debtor is a shiftless person and, moreover, is full of vanity and conceit. The creditor’s point in making the demand might be simply to get the debtor to quit his proud behavior and to humble himself.
God deals with people the same way. By serving notice on me that I have to obey all His Commandments, God leads me to realize that I cannot meet any obligations––no matter how hard I try. Once He has humbled me, He approaches me with the Gospel. Modern preaching lacks this concept that the natural heart must be humbled. When a person says to a preacher, “I simply cannot believe,” and the preacher tells him, “Oh yes, you can. All you need is the earnest desire to believe. You can get rid of your sins. All you have to do is to struggle with them,” that is a horrible way to preach.
Law & Gospel: How to Read and Apply the Bible by C. F. W. Walther (A Reader’s Edition)