As dirt farmer posted paul understood the old testament from a to z. Paul was in a unique place to show how the old and the new mesh and fit together.
After the death of Christ we no longer follow the Commandments of Moses,but are shown how Israel failed to follow God. I believe Paul said that it was because of the hardness
of their hearts.
Paul went on to say that the Gentiles (that's us) aren't under the law at all but under grace.
Ro 6:14 For sin shall not be your master, because you are not under law, but under grace.