Then they said to him, "What must we do, to be doing the works of God?" Jesus answered them, "This is the work of God, that you believe in him whom he has sent."
Martin Luther concisely expounds on the "two words of God"--showing how the Law of God is intended to bring about desperation, a kind of desperation that only the Gospel of God can deliver us from. The Law reminds us that we're a lot worse than we think we are; the Gospel tells us that God's grace is infinitely bigger than we could have ever imagined.
Now, when a man has learned through the commandments to recognize his helplessness and is distressed about how he might satisfy the law--being truly humbled and reduced to nothing in his own eyes--he finds in himself nothing whereby he may be justified and saved.
Here the second part of Scripture comes to our aid, namely the promises of God which declare the glory of God, saying, "If you wish to fulfill the law, come believe in Christ in whom grace, righteousness, peace, liberty, and all things are promised to you. If you believe, you shall have all things; if you do not believe, you shall lack all things."
Martin Luther, On Christian Liberty