The old testament was man showing God outwardly his commitment to God. They were told to circumcise to show God they were faithful. Seeing how we all sin and needed a savior and how by our works we cannot be saved, Jesus died on the cross for us. So, Jesus said rather than circumcising our body we need to do it to our heart. Jesus is about the unseen, our hearts, our faith. That's why jews hate Christians because we don't have to do anything except love Jesus and others, repent and pray and do good things for the kingdom.