Today's Christians often err on either side of extremes concerning the law and the Christian. On the one hand, Christians who fear being legalistic disregard the law and say that it has no use to the Christian life- this is called Antinomianism. The other extreme is that people are justified by doing the law- they believe that they are saved through the keeping of the law. This is called Legalism.
The historic Protestant position is that the Christ justifies the sinner by grace, based on NOTHING OTHER THAN GRACE. When one is justified he then begins to keep the law out of gratitude and love for Jesus Christ. Jesus said, If you love me, you will keep my commandments.
1 John 2:3-4 And hereby we do know that we know him, if we keep his commandments. He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
But there is no one who in everything keeps them; there would thus be no knowledge of God in the world. To this I answer, that the Apostle is by no means inconsistent with himself; since he has before shewed that all are guilty before God, he does not understand that those who keep his commandments wholly satisfy the law (no such example can be found in the world;) but that they are such as strive, according to the capacity of human infirmity, to form their life in conformity to the will of God. For whenever Scripture speaks of the righteousness of the faithful, it does not exclude the remission of sins, but on the contrary, begins with it.
But we are not hence to conclude that faith recumbs on works; for though every one receives a testimony to his faith from his works, yet it does not follow that it is founded on them, since they are added as an evidence. Then the certainty of faith depends on the grace of Christ alone; but piety and holiness of life distinguish true faith from theft knowledge of God which is fictitious and dead; for the truth is, that those who are in Christ, as Paul says, have put off the old man. -Calvin Commentaries