MDB 2: Matthew 5
"Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them" (Matt. 5:17).
It is unwise to attempt to pin down what "fulfill" means in this breathtaking statement by Jesus. For starters, "the Law" may well be fulfilled in a different way to the "Prophets." But this much may be said: Jesus fulfilled all the demands of the law. As the Mediator and our substitute, he perfectly obeyed every precept and bore every penalty. In that sense, he "fulfilled the law." But Jesus means more than this here. Jesus is saying that the law is now being fulfilled in those whom he has forgiven. Having been justified, Christians must now pursue holiness. Sanctification involves obedience to God's law: "the righteous requirement of the law" is to be "fulfilled in us," Paul says of those who "who walk not according to the flesh but according to the Spirit" (Rom. 8:4). This is not legalism, but a view that sees a continuing role of the law in the lives of those who have expressed their inability to keep it, casting themselves wholly upon the grace of God in the gospel and obedience of Another - Jesus - in fulfilling the law's strict demands. This "third use of the law" (as Calvin expressed it) views Jesus in Matthew 5:17-48 as expounding the continuing relevance of the law in the lives of Christians. The Sermon on the Mount is not meant to be viewed as another "performance-based" ethic - we saved by grace and sanctified by works. That would be to fall foul of Paul's question to the Galatians: "Having begun by the Spirit, are you now being perfected by the flesh?" (Gal. 3:3).
The motivation of obedience by those justified by faith alone in Jesus Christ alone is not to impress God. Christians pursue holiness because of who they are - what they are - justified, forgiven, ransomed, healed, restored sinners. The motivation is not a desire to receive merit but one of gratitude for grace already received. Our necessary obedience as those in union with Christ must repudiate any semblance of self-justification in the interests of a grace-driven, gospel-centered, Christ-exalting ethic.