Take a look at Romans (2&3 I believe). For several verses Paul says things like "The doers of the law will be justified" & each man being judged according to his works. Then Paul goes on to talking about Faith being necessary for salvation.
The protestant position is usually: These are two plans of salvation. That is, plan one, you can be saved if you keep the law completely, but nobody keeps the Law. Plan two is that Jesus kept the law for you, so you put your faith in him for salvation.
The Catholic position is that there is but one plan of Salvation: You must keep the Law (obedience) in order to be saved, but only through Christ (faith) can you keep the law in any way that God recognizes and only through Christ can you be forgiven of your failure to keep it completely.
Now... if that is a reasonable summary of the positions... Which makes more sense? That God instituted TWO plans of salvation (one of which was a null-set with no participants)? Or ONE plan of Salvation throughout all time?