Matthew 5:21-48 Jesus is comparing behaviour and heart. The phrases we see are "You have heard it said..." and "But I say". What Jesus is doing it comparing the letter of the law with the spirit of the law. For example, Matthew 5:21 says " You have heard it said that it was said to those of old, 'You shall not murder; and whoever murders will be liable to judgement.'
This is talking about the law we find in Exodus 20:13, the Ten Commandments.
Jesus goes on to unpack that more, and draws out the spirit of that command
Matthew 5:22-25
But I say to you that everyone who is angry with his brother will be liable to judgment; whoever insults his brother will be liable to the council; and whoever says, ‘You fool!’ will be liable to the hell of fire. So if you are offering your gift at the altar and there remember that your brother has something against you, leave your gift there before the altar and go. First be reconciled to your brother, and then come and offer your gift. Come to terms quickly with your accuser while you are going with him to court, lest your accuser hand you over to the judge, and the judge to the guard, and you be put in prison.
Now we see the spirit behind the law, and the spirit of the law deal with our heart. What is really going on in our hearts. We know that anger is behind many many murders. It's is unlikely someone woke up one morning and decided to murder another person, no, there was something sinister bubbling away in the heart that finally manifests itself in the act of murder. It's what goes on in the heart that Jesus was dealing with.
Any one who reads the Ten Commandments will quickly deduce that these commands are impossible to keep. Yes, some can keep a couple of the commands, but not all, and, according to scripture if you break one command, you are guilty of breaking them all. In a nutshell, mission impossible!
Jesus' sermon on the mount makes us look at the state of our hearts, so it becomes a lot more than trying to follow a list of rules. The bible says, of our hearts:
Jeremiah 17:9
The heart is deceitful above all things,
and desperately sick;
who can understand it?
Not good!
On one hand is a list of righteous commands, on the other is a desperately sick and deceitful heart trying to follow those righteous commands. An impossible ask, yet the whole purpose of Jesus pointing this out was to show that In Him would be the fulfilment of what God said through Jeremiah
Jeremiah 31:33
For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people.
Making Jesus Lord of our hearts fulfils this prophesy, for we receive a new heart In Him, and this takes the letter of the Law and the Spirit of the Law, meshes it together with a new heart to live it out. Thank God for Jesus.
