EJ quoted Prov. 21:3:
To do what is right and just
is more acceptable to the Lord than sacrifice.
It's important to note that not all sacrifices were acceptable to God. If a sacrifice was given without sincere repentance and if a person's life didn't reflect that repentance, then God was not pleased with the offering. Consider Isaiah 1:11 which reads:
The multitude of your sacrifices—
what are they to me?� says the Lord.
I have more than enough of burnt offerings,
of rams and the fat of fattened animals;
I have no pleasure
in the blood of bulls and lambs and goats.
If we read that verse in context, we see that the people are making sacrifices, but they are living sinful lives. Obviously, there is no repentance behind the sacrifices, making them meaningless in God's eyes. God wants them to live godly lives of repentance and obedience rather than offer worthless sacrifices to him.
And, in fact, if we look at the preceding verse in Proverbs 21, we read this: "A person may think their own ways are right, but the Lord weighs the heart." So we see that, in context, this is exactly what Solomon is talking about here. He's talking about the arrogant who do what they want apart from the Lord. Sacrifices made by such people are meaningless to God. Put another way, religious acts are without value apart from righteous living.
So this proverb isn't saying that people don't have to make sacrifices to atone for their sins. It's saying that sacrifices made with an impure heart aren't acceptable to God. Cain is a perfect example of this. See here:
http://www.letusreason.org/Doct100.htm
And there's Psalm 51:16-17, which reads, "For you will not delight in sacrifice, or I would give it; you will not be pleased with a burnt offering. The sacrifices of God are a broken spirit; a broken and contrite heart, O God, you will not despise."
Again, it's about one's heart attitude. Prov. 21:3 isn't talking about salvation at all.