"For we are not under the Law, but under Grace." (Romans 6:14)
Many Christians use this verse to say that we cannot use the Law to correct the behavior of fellow Christians. I differ (I do not beg).
On the contrary, in the same Letter, the Apostle Paul writes, "Owe no man anything, but to love one another; for he that loveth another hath fulfilled the law. For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself. Love worketh no ill to his neighbour: therefore love is the fulfilling of the law."
Furthermore, Jesus agreed with the man who said, "there is one God; and there is none other but He: and to love Him with all the heart, and with all the understanding, and with all the soul, and with all the strength, and to love his neighbour as himself, is more than all whole burnt offerings and sacrifices. "
Jesus Himself said, "'Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.'--This is the first and great commandment. And the second is like unto it, 'Thou shalt love thy neighbour as thyself.' On these two commandments hang all the law and the prophets. "
So if our definition of love does not include the message of the Law and the Prophets, then we lack the kind of love that Jesus wants us to have for Him and for one another. That's pretty serious.
The point of Paul's statement is that we no longer need to fear the punishment due us for breaking God's Law. But in no way is he suggesting another set of standards for Christians to live by than God's Holiness, which is revealed by His Law. That's why he comes back to it in the later passage. God wants us to follow His Commandments, not because He'll punish us if we don't (though He will chastise us for disobedience, Hebrews 12:1-11), but because He has been so gracious to save us, that we will do anything for Him (1 John 4:19; John 14:15, 24).
Every New Testament command has its roots in the Old Testament, the Law and the Prophets. To call them obselete is to abuse one verse of Scripture in trying to create a whole doctrine.