The law and the words of the Prophets that make up the Old Testament are the WORD of God. Not to be dismissed or disregarded because of grace. The Lord wanted us to know that He didn’t come to abolish it, but to fulfill it, until ALL is fulfilled… We, who are His followers/ disciples have this law written into our hearts and minds and are given the Holy Spirit to help us navigate through this life. We need this. We need to obey Jesus’s commands - to love and to obey the law.
Grace is what Jesus gave us because it’s impossible for us not to fail or fall down. His grace is enough for us to keep going in spite of it and because He already paid for our sins - we don’t have to pay the price. But a sin is still a sin, even under grace.