The Law as a Covenant
The law as a covenant says, “Do this and live” (Lev 18:5; Ez 20:11; Lk 10:28; Rom 10:5; Gal 3:12). It demands perfect obedience for eternal life (Gal 3:12; 5:3). It makes no provision for forgiveness of sins (Gal 3:10). The law covenant is inflexible and absolute. Even one sin against the law covenant brings guilt and eternal condemnation. That means we're all condemned in the court of the law covenant because we're all sinners. The good news is that Christ's perfect obedience to the terms of the law covenant brings justification and eternal life for all who belong to Him.
If, however, we forget the law covenant's strict requirement of perfect obedience for justification and eternal life, then we'll inevitably start to think that we can imperfectly keep the law for our justification and eternal life. This isn't theoretical. While I appreciate many of the things N.T. Wright says, I believe he's wrong about this in particular. Wright, popular among many evangelicals, teaches that we initially receive justification and life by grace, but we retain our justification and life by a kind of imperfect soft-obedience to the law. Wright, and those who follow him, have forgotten the strict demands of the law as a covenant.
The Law as a Standard or Rule
In Christ, we're free from the law as a covenant! But we're not free from the law as a standard or rule. After Jesus justifies us, He graciously points us to His good law as our guide in sanctification. As believers, we express our love for Christ by learning to keep the standard of His law more and more. The law of God is the Christian's “rule of walking” faithfully in Christ. Romans 7:12 says, “The law is holy, and the commandment is holy and righteous and good.” Paul says, “I delight in the law of God in my inner being” (Rom 7:22). Romans 8:4 says that Christ satisfied the law so that “the righteous requirement of the law might be fulfilled in us.”
If we don't emphasize the law as our sufficient standard of faithful conduct, then we'll start to make up our own standards. Preachers will teach “practical” ways of applying the gospel that aren't anchored in God's law but only in their own experiences and preferences. That's authoritarianism. Church cultures, rather than God's law, will tell us how to live in light of the gospel. Extra-biblical emphases and practices will arise by the “leading of the Spirit,” while God's own law is marginalized. If young evangelicals don't emphasize and apply the biblical doctrine of God's law, they'll inevitably be re-enslaved to the legalism from which they were liberated.
The Bible teaches “through the law comes knowledge of sin,” (Rom 3:20), “where there is no law, there is no transgression” (Rom 4:15), and “sin is not counted where there is no law” (Rom 5:13). The doctrine of Christian liberty is based on the Bible's doctrine of the law. If we lose the doctrine of law, then we'll lose our liberty. We will become legalists again. But when we emphasize the law of God, we'll be free from all extra-biblical commandments to walk wisely in light of His sufficient commands.
The gospel alone isn't enough to keep us from legalism. The law of God, correctly understood as a covenant and a standard or rule, is a necessary and powerful protection from legalism.