I was reading your question and answer about the Rich Young Ruler and it got me to thinking. Since Jesus had not yet died and been resurrected, then technically the Rich Young Ruler as well as everyone else was still under the Old Testament Law. Therefore, that is why Jesus gave him a works based answer to his question about eternal life. I guess my question to you is, When did the New Covenant actually begin?
I don’t believe Jesus taught a works based salvation to anyone. He repeatedly emphasized belief in Him as the basis for our salvation (John 3:16, John 6:28-29, John 6:38-40), and said even the level of obedience to the Law demonstrated by the Pharisees would be insufficient to save them (Matt. 5:20).
The first appearance of the phrase New Covenant is in connection with the Last Supper, but I believe the Lord was teaching its principles from the beginning of His ministry. Remember, God had promised the New Covenant to Israel 600 years before the Lord came (Jere. 31:31-34) and He was intended to be the fulfillment of that promise. They haven’t received it yet because they rejected it.