First, it’s important to understand what Old Testament (OT) law is…
There are hundreds of laws found in the OT books Exodus to Deuteronomy, but not all laws are created equal. Out of all those laws, 10 of them have a special place – the 10 commandments. The 10 commands were spoken directly by God to Israel from the fire and the cloud that covered the mountain (Ex. 19:16-18), and the rest of the laws were given by God through Moses. In fact it’s fair to say that all the other laws are really just breaking down the 10 commands into specific cases for a particular group of people (Israelites), for a particular time in history (before Jesus), and in a particular place (Israel)…
Here's an example: Take the 6th command “Do not murder”… What happens if I kill someone accidently? Let's say, ummmm… I don't know… because I'm chopping down a tree with my axe, and the axe head flies off the handle and it kills someone? Well Deuteronomy 19:1-7 tells you what to do. It's not really a new law, just a particular application for the 6th command.
Jesus changes the way the law is to be applied to Christians today
Christians are a different group of people, living in a different part of history (after Jesus’ death & resurrection), living in a different place (everywhere on earth). And Jesus changes the way the law is to be applied to Christians today because he is the 10 commandments.
Jesus is a living, breathing, walking, talking, human example of the 2 great commands to love God and love your neighbour. Jesus is the fulfillment of the Law (Matt. 5:17-20). If you want to know what it would be like for someone to actually love God with all their heart, soul, mind, & strength, and to love their neighbour as they love themselves, and to keep all the 10 commands… then look at Jesus.
• All of the OT commands can be summarised in just 10 commands
• And the 10 commands be summarised in just 2 commands (Matt. 22:37-40)
• And those 2 commands can be summarised in 1 person – Jesus.
So if Jesus fulfills the OT law, what should Christians do with it?
If you’ve been asking this question for a while, you’re in good company! Christians have been asking this question ever since the ascension of Jesus.
In Acts 15 the apostles form a council in Jerusalem to sort out this question because “certain individuals came down from Judea to Antioch and were teaching the believers: 'Unless you are circumcised, according to the custom taught by Moses, you cannot be saved.’”
So should Gentile (non-Jewish) Christians and Jewish Christians obey the law of Moses (OT law) now they follow Jesus?
There is some discussion, but the short answer is no (Acts 15:28-29). And it’s clear in other places in the new testament that we aren’t under the law of Moses but under the law of grace (have a look at Romans 6:14, Galatians 2:19-21, Ephesians 2:11-22). So you’re free to eat bacon, wear clothes made of 2 types of material, and live in a mouldy house (all things you couldn't do in the OT); but there is still a place for the 10 commands in the Christian life.
You see, even though you don’t have to follow the 10 commands – because you are one with Jesus and he is the 10 commands – they still show us how we should relate to God and to each other. That’s why the apostle Paul can tell us that it is right for us to follow the 5th commandment when he quotes it in Ephesians 6:1-3.