Throughout the history of the church, practically every time there has been a major advance in clarity and understanding that brings some key gospel truth into clear focus, almost immediately some liberalizing tendency seeks to nullify the gain.
In the eleventh century, for example, Anselm of Canterbury untangled a millennium of confusion about the atonement in his landmark book Cur Deus Homo. Anselm demonstrated from Scripture that Christ's death was an offering to God—a full satisfaction rendered by Christ to God—rather than a payment to appease Satan or an act of martyrdom at the hands of wicked men. Anselm's work highlighted the simple but crucial truths that the atonement was the central reason God became a man, and that redemption is a gracious work of Christ to be received by faith alone. But no sooner had the truth of divine grace begun to regain its rightful place at the center of the gospel than Peter Abelard rose up and denied that Christ's death was any kind of payment or substitution at all. In Abelard's view, the cross was merely a radical example of self-sacrificial love, designed to win men's hearts and give them a pattern to follow. Abelard's explanation of the atonement (a classic expression of liberal thinking) thus made the work of redemption something the sinner must do for himself by mimicking Christ.
Nevertheless, Anselm's work paved the way for the Protestant Reformation 500 years later. But even the Reformation was immediately answered with a new liberalizing tendency, Socinianism. Named for Laelius Socinus (an inveterate skeptic who nevertheless retained a form of religion), Socinianism was a typically misguided liberal attempt to retain the moral essence of Christ's teaching while rejecting virtually every orthodox doctrine and supernatural element of the Christian faith, beginning with the authority of Scripture.
Socinianism was followed by a string of similar liberal movements. Most of them appeared in reaction to various revivals and expansions of gospel preaching. The Great Awakening in colonial America was followed by the rise of deism. The Second Great Awakening was severely marred by an upsurge of moralistic free-will theology and perfectionist dogmas. That in turn was exacerbated by a major movement toward Unitarianism in the early nineteenth century. Meanwhile, in Germany, Friedrich Schleiermacher was blending Enlightenment philosophies with Christian moral teachings, using higher criticism to justify skepticism about the Bible's supernatural claims.
Modernism managed to smuggle practically every classic expression of theological liberalism into evangelical circles under the guise of staying abreast of the times. Virtually every significant evangelical institution that embraced any degree of modernism soon abandoned evangelical principles. And practically all of them became empty shells of what they once were.
The legacy of such movements is clear—or it ought to be. No good has ever come from the liberalizing tendency. It is rooted in a way of thinking that is hostile to the authority of Scripture; it inevitably corrupts the simplicity of the gospel of grace; and it fosters skepticism and (in the worst cases) rank unbelief.