The Law Combats Legalism
Joel Beeke and Mark Jones, writing in A Puritan Theology (p. 570):
The third use of the law combats both Antinomianism and legalism.
Antinomians wrongly appeal to justification by faith alone, which, though granted apart from works of the law, does not preclude the need for sanctification. One of sanctification’s most important elements is the daily cultivation of grateful obedience to the law.
Moreover, neglect of the third use of the law can result in legalism, and often does, for we cannot live without law. When, as an alternative to God’s law, an elaborate man-made code is developed for believers to follow, covering every conceivable problem and tension in moral living, no freedom is left for believers to make personal decision based on the principles of Scripture. In such a context, man-made law smothers the divine gospel, and legalistic sanctification swallows up gracious justification. The Christian is brought back into bondage akin to that of medieval Roman Catholic monasticism.
Equally enslaving is the freedom that allows the Christian to follow his own emotions and impulses. Healthy Christian spirituality arises from careful meditation upon the principles of the law of God combined with heartfelt consecration to do the will of God (Rom. 12:1-2).