So then, the law was our guardian until Christ came, in order that we might be justified by faith.
The law and the gospel
What is the difference between the moral law and the Gospel? The law requires that we worship God as our Creator; the Gospel, that we worship him in and through Christ. God in Christ is propitious; out of him we may see God’s power, justice, and holiness: in him we see his mercy displayed.
The moral law requires obedience, but gives no strength (as Pharaoh required brick, but gave no straw), but the Gospel gives strength; it bestows faith on the elect; it sweetens the law; it makes us serve God with delight.
Of what use then is the moral law to us? It is a glass to show us our sins, that, seeing our pollution and misery, we may be forced to flee to Christ to satisfy for former guilt, and to save from future wrath.
But is the moral law still in force to believers? Is it not abolished to them? In some sense it is abolished to believers, in respect of justification. They are not justified by their obedience to the moral law. Believers are to make great use of the moral law, but they must trust only in Christ’s righteousness for justification. Noah’s dove made use of her wings to fly, but trusted only the ark for safety. If the moral law could justify, what need was there of Christ’s dying?
— Thomas Watson, The Ten Commandments, pp. 33-34 (B.T.)
From Daily Devotions from the Puritans – the 1997 Bryntirion Press book by I.D.E. Thomas