Romans chapter 4

vs 1

We get today a case study on the man Abraham. Abraham's an important person, because he was the receiver of the covenant of circumcision (before the OT law was even written).

vs 2

This is a complicated little sentence. Is Paul saying that even if Abraham was justified by his works, that he still wouldn't be able to boast before God? That's how you want to read it, but that doesn't seem to be the argument that Paul is following. Paul's argument goes on to prove that Abraham did not get justified by his works anyway, so it's sort of beside the point. Perhaps Paul is throwing in that little aside as a sideways glance at the Jews. It's a logical argument - if Abraham was justified by works, he is still in effect justified by God. But Paul doesn't pursue it.

vs 3

Instead, Paul wants to show that Abraham had his righteousness credited to him. For someone who is most probably making his scriptural arguments entirely from memory, he is placing a lot of value on one or two specific words.

vs 4

Remember that, when you're paying someone. You owe it to them to pay them. This is the way of the world, though, that is pretty much understood in order that mercantilism can continue.

vs 5

So righteousness is not earned as a wage, but it is credited to you like a gift. I think the "does not work" is not imperative - that is, I don't think you have to not work in order to trust God. But you can't trust work.

vs 6

Paul now finds another source, highly valued by the Jews - David. As far as Paul is concerned, David is saying the same thing.

vs 7-8

David obviously doesn't say anything that is as clear as what Paul is making out from these two verses. However, if you knew the whole psalm, you would know that David says that he confessed his sins to God, and God forgave them. The coverings of sins does not come from working hard at obeying the law. It comes from trusting God to forgive those sins.