Greetings, Alex!
The strongest description of New Testament grace that I have ever read personally came from a Messianic Jew, and his answer was that the grace of God (including the outpouring of the Holy Spirit) is bestowed upon us so that we might be empowered to walk in obedience. Seen from this perspective, the grace of God is not in conflict with such verses. Old Testament saint or New, Jew or Gentile, He calls us all to walk in obedience to Him. But His mercy is also new every morning, should we slip. He only asks that we get up and start striving for obedience once more, and lean upon His empowerment to fulfill His will in our lives.
As for the rich man, he was under Old Testament law as a Jew at the time, so obedience to God for him involved keeping the Jewish law. In the New Covenant, the question simply becomes how much of that law is still required of New Testament believers, Jew or Gentile. I must confess I have no idea what atheists and Jehovahs Witnesses make of Jesus' words here, but those who are distant from God place all sorts of requirements upon men that are NOT of importance to Him, while neglecting to teach them the things that truly are. This has been that pattern of religious leaders who are distant from God for millennium now (Matthew 15:9-14).
