Obsolete for salvation yes. Not obsolete for measuring our actions, as God's children, to know what is sin, to know what to do, to sin no more. Without knowledge of God's will, his commandments,
his law, written on our hearts, under his new covenant, how will we as his saved children know what is sin? Obedience to God's will, God's commandments, God's law, follows receiving grace of salvation through his gift of faith.
John 8 (ESV)
3 The scribes and the Pharisees brought a woman who had been caught in adultery, and placing her in the midst
4 they said to him, "Teacher, this woman has been caught in the act of adultery.
5 Now in the Law Moses commanded us to stone such women. So what do you say?"
6 This they said to test him, that they might have some charge to bring against him. Jesus bent down and wrote with his finger on the ground.
7 And as they continued to ask him, he stood up and said to them, "Let him who is without sin among you be the first to throw a stone at her."
8 And once more he bent down and wrote on the ground.
9 But when they heard it, they went away one by one, beginning with the older ones, and Jesus was left alone with the woman standing before him.
10 Jesus stood up and said to her, "Woman, where are they? Has no one condemned you?"
11 She said, "No one, Lord." And Jesus said, "
Neither do I condemn you; go, and from now on sin no more."
Romans 2 (ESV)
14 For when Gentiles, who do not have the law,
by nature do what the law requires, they are a law to themselves, even though they do not have the law.
15 They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them
16 on that day when, according to my gospel, God judges the secrets of men by Christ Jesus.
Hebrews 8 (ESV)
10 For this is the covenant that I will make with the house of Israel after those days, declares the Lord:
I will put my laws into their minds, and write them on their hearts, and I will be their God, and they shall be my people.