Christ never taught that. This is where +Paul is literally building a new religion.
Sure Christ taught that. Right through Paul. The Bible is either infallibly correct or it is not.
We have here the authority Moses claims from his encounter with God on Mt Sinai, and +Paul who claims his authority from an encounter to Damascus. +Paul is saying the Law is worthless. That was "new" Judaism.
Paul is saying that the Law is worthless toward salvation, not that it has no value. Christ said the Law had great value. Was David saved because he kept the Law, or because of his faith?
That's not the only possibility.
Because he repented.