Martin Luther said, "Satan, the god of all dissension stirs up daily new sects. And last of all which of all others I should never have foreseen or once suspected, he has raised up a sect such as teach that men should not be terrified by the law, but gently exhorted by the preaching of the grace of Christ."
Luther also said, "The first duty of the Gospel preacher is to declare God's Law and show the nature of sin."
Charles Spurgeon said, "I do not believe that any man can preach the gospel who does not preach the Law."
Spurgeon also said, "Lower the Law and you dim the light by which man perceives his guilt; this is a very serious loss to the sinner rather than a gain; for it lessens the likelihood of his conviction and conversion. I say you have deprived the gospel of its ablest auxiliary [its most powerful weapon] when you have set aside the Law. You have taken away from it the schoolmaster that is to bring men to Christ... They will never accept grace till they tremble before a just and holy Law. Therefore the Law serves a most necessary purpose, and it must not be removed from its place."
More Spurgeon; he said that sinners "must be slain by the law before they can be made alive by the gospel."
Jonathan Edwards said, "The only way we can know whether we are sinning is by knowing His Moral Law." As an aside, have you ever read his sermon "Sinners in the Hands of an Angry God?" Edwards definitely understood and put into practice the Biblical principle of "Law to the proud, and grace to the Humble."
John Wesley instructed a young evangelist to "Preach 90 percent law and 10 percent grace."
Wesley also said, "it is the ordinary method of the Spirit of God to convict sinners by the Law. It is this which, being set home on the conscience, generally breaketh the rocks in pieces. It is more especially this part of the Word of God which is quick and powerful, full of life and energy and sharper than any two-edged sword."
Another Wesley quotation: "Before I can preach love, mercy and grace I must preach sin, law and judgment."
The great revivalist George Whitefield wrote, "First, then, before you can speak peace to your hearts, you must be made to see, made to feel, made to weep over, made to bewail, your actual transgressions against the Law of God."
C.S. Lewis wrote, "When we merely say that we are bad, the 'wrath' of God seems a barbarous doctrine; as soon as we perceive our bad-ness, it appears inevitable, a mere corollary from God's goodness..."
John MacArthur wrote, "God's grace cannot be faithfully preached to unbelievers until the Law is preached and man's corrupt nature is exposed. It is impossible for a person to fully realize his need for God's grace until he sees how terribly he has failed the standards of God's Law."
John Newton (who wrote "Amazing Grace") warned, "Ignorance of the nature and design of the Law is at the bottom of most religious mistakes." Indeed it is.
John R. Stott wrote, "We cannot come to Christ to be justified until we have first been to Moses, to be condemned. But once we have gone to Moses, and acknowledged our sin, guilt and condemnation, we must not stay there. We must let Moses send us to Christ."