"You have heard it said... but I say to you..."

Ever wonder why Jesus says this? It sounds at first as if He's contradicting the law of Moses. Intimating that the law got it wrong, but "here's what you should REALLY do." Indeed, that's probably what the Pharisees and the strict followers of Moses thought - it was even shocking for His own disciples to hear no doubt.

Here's some helpful commentary on this topic from Lee Irons in his article A New Locus of Authority:

"Perhaps an analogy may help. The constitution of the United States and the constitution of England may have many points in common. I don’t know for sure, but I wouldn’t be surprised if the constitution of England (even if based on common law) contained similar provisions concerning civil liberties such as the right to freedom of religion or freedom of speech. But are citizens of the United States bound by these provisions in the constitution of the England? No. We fought a Revolutionary War, broke from England, and we are now bound by the constitution of the United States. The fact that there are points of similarity and continuity doesn’t change that fundamental legal fact. We can trace lines of organic, historic continuity from the Magna Carta to the Bill of Rights. But, legally, we Americans are not under the Magna Carta or the constitution of England.

So with the new covenant. We Christians are members of a new polity called the new covenant. We are not bound by the constitution of Israel (the Mosaic Law). We are not Jews. Are there points of similarity and continuity between the old covenant and the new? Of course. Every moral teaching contained in the old covenant is carried forward into the new. Christians are still obligated to flee idolatry, sexual immorality, murder, and so on. If anything, our view of what is entailed by God’s moral law is expanded and heightened in the new covenant. The prohibition against murder entails a prohibition against hating our neighbor in our heart. The prohibition against adultery includes the prohibition against lust. Neighbor-love is expanded to enemy-love. And so on. But in reality, even these heart attitudes were always a part of the moral law. They are just more clearly seen in Christ and made more explicit through his teaching."

Read the whole article