There are three sets of laws in the scope of our conversation, according to the Catholic Church. First, there is natural law; this is the law "written in the hearts of men" (Romans 2:15) that men obey due to the call of their conscience and natural reason. No one needs to give people a law that would say that God is to be loved, men naturally know that it is reasonable that they are to love their creator. When they fail at loving God they fail not because they were not given that law, but because they do not follow their conscience under stress. This state of affairs allowed Noah to love God even before any dispensation of law occurred. He was naturally perfect. (Abel is another example of such natural love of God).
Second, there are the Noachide commandments, usually summarized as 7 in number, that were given Noah (Gen 9); they are not as strong as the Ten commandments given Moses. Unlike natural law, these are codified, precise instructions. In His loving kindness God gave them so that men can find it easier to properly form their consciences.
Thirdly, there are the Ten Commandments given Moses, and through Moses, to the Jews. They develop and strengthen the Noachide commandments. Then more instruction followed, the total number of commandments given the Jews is just over 600.
So these are three different things, and the Natural Law underlies all the precise commandments. Note that we, as Christians, obey the Ten Commandments not because they were given through Moses to the Jews (if we did, we'd have to obey all 600+), but because they are also natural law. Jesus repeated most of them and explained them, and brought them out of the realm of jurisprudence back into the realm of human conscience and its proper function of loving God(Matthew 5-7).
are you reading Genesis 6:9 literally or as a metaphor?
I read it literally, but I understand the idiomatic usage of "walk". I think that it is clear that the inspired author wanted to tell us that Noah was a perfect, sinless man, and also to understand that one becomes perfect by "walking" with God, that is, doing everything as if God is right next to you.
So Catholics believe the Israelite's understanding of the Atonement, animal sacrifice and "repentance" were incorrect or insufficient?
Yes, insufficient. They were correct because God gave them to them, but they were given them in order to prefigure for them the perfect sacrifice of Christ and atonement of sin that flowed therefrom, and teach them repentance. This is what plan, or "economy" of salvation is all about: the gradual revelation of God, initially to the Jews and now to all nations. It is not unlike rearing a child: initially you giove a child simple rules and punish him for violating them. But as the child grows, your plan evolves: you rely less on the rules and more on the intellect of the child, so that out of a child an adult is formed.
How about the human understanding of murder, theft, adultery, etc. prior to the Noachide laws and the Ten Commandments? Were these not sins until formalized by God? That is that from Adam to Noah no man could be condemned for these sins.
Are you saying that Noah and Moses received new revelation not previously known to mankind? How would this affect the sinning by those outside of the Israelite context, i.e. Chinese, Egyptians, the whole rest of the world. Are they not under condemnation for sin?