The Law is God's Word. But the Law is essentially an uncomfortable word (which is what we need to understand). God hasn't given His Law as a way of salvation. Romans 3:20 explains what is and what isn't the function of the Law:
'Therefore by the deeds of the law no flesh will be justified in His sight, for by the law is the knowledge of sin.'
In the earlier verses of Romans 3 we read that no one has kept the law, no one is righteous (vv.10-18). In fact, when the Law is heard, it revels to people their guilt (Romans 3:19). That's the job of the Law. God has not spoken His word of Law as a way of salvation: it cannot justify. God has spoken His Law in order to show us our sin. The Law opens our eyes to see our guilt.
That's why I said the Law was an uncomfortable Word. It isn't there to encourage us in what we need to do (for we cannot 'do' anything to earn God's favour); no, it's there to show us what we haven't done (or what we have done that we shouldn't have). The Law, in the Words of the Prayer Book confession, shows us that 'We have left undone those things which we ought to have done; And we have done those things which we ought not to have done; And there is no health in us.' So the Law is a humbling Word.
The Law reveals sin, and so the Law brings condemnation. The Law is an uncomfortable Word because it threatens sinners with punishment and shows us how what we deserve from God is His wrath.
But God speaks a second Word: His Word of Gospel. Where the Law was an uncomfortable word, the Gospel brings comfort. Where the Law was a threatening Word, the Gospel brings grace, forgiveness and joy. Where the Law kills, the Gospel brings life.
The Law tells us about ourselves, and what it tells us isn't good. It tells us how we've disobeyed. It tells us how we've missed the mark. It shows us the depths of our sin, and it shows us our just desserts.
But the Gospel tells us, not about ourselves, but about Jesus! The Gospel tells us how He obeyed perfectly for us. The Gospel tells us about how He lived the perfect life in our place. The Gospel tells us about how Jesus bore the wrath of God which we deserved in His death on the Cross. And the Gospel tells us how He has been raised to life and ascended to the Father's right hand on our behalf. The Gospel isn't about us, it's about Jesus: and that's why the Gospel is good news!
And that's why we mustn't confuse Law and Gospel. For the law is uncomfortable, but the Gospel is full of comfort, peace and joy. The Law kills, but the Gospel gives life. The Law is all about us, but the Gospel is all about Jesus.
The Law shows us what we haven't done, can't do and won't do. But the Gospel shows us what Jesus has done and has completed. The Law says 'Do!', but the Gospel says 'Done!'