To show us our need for grace, law must come first, for the law shows me my sinful and helpless condition, and then I understand my need for grace.

On the other hand, to be saved from my helpless condition, grace must come before law. No amount of good-doing or law-keeping will make up for my sins—even my most righteous deeds are filthy rags in the eyes of God (Is 64:6).

To see my need for salvation, law comes first. But to receive salvation, grace comes before law.

But what about my sanctification, my daily growth and becoming like Jesus? Does law come first? Many Christians would think so. They would say: you need grace to get saved, but once you are saved, you now focus on obedience (law). This is both dangerous and false.

It is dangerous because it tends to put the power of obedience on the shoulders of Christians instead of God's Spirit who alone can enable them. It is dangerous because it tends to promote a false view of God: My obedience earns His acceptance. This nurtures pride when the believer obeys and discouragement when he falls. Consequently, his life become a roller coaster.

The thinking is also false because the New Testament teaches the opposite order: grace that enables and motivates obedience.  

The grace that saved me is the same grace in which I now stand. That’s why Paul wrote to the church in Corinth, “Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand” (1 Cor 15:1). The gospel they “received” (justification, moment of conversion) is the gospel in which they now “stand” (sanctification, daily growth). That’s why Paul spends three chapters doxologizing God for His amazing grace in Ephesians 1-3 and then, based on that immovable foundation, he writes in chapter four verse one, “Therefore...walk in a manner worthy of the calling with which you have been called” (Eph 4:1). For the same reason, every command in the New Testament letters is grounded in the gospel itself. As Tim Keller so wisely teaches, the gospel is not only the entry point to Christianity, it is its essence!

Should Christians obey? Yes! But obedience is only possible by the grace and power of the gospel. It goes back to Dr. T. J. Betts' comment (as well as Keller’s, Ligon Duncan’s, and many, many others): “I am accepted, therefore I obey.”