I've been studying Romans for the past 2 weeks and at first ironically after reading chapters 3-4 I felt the Protestants were absolutely correct on Justification by Faith Alone, now after reading chapters 5-8 I am almost certain the Catholics are correct in their explanation.
The way I understand it, the Faith alone issue is almost a misunderstanding. We have such close ideas on justification it's amazing how different people think they are. Justification for the Protestant is by faith alone, but they don't REALLY mean faith alone, if you asked a Protestant does someone have to repent of their sins? yes they would answer. Do they have to love God? yes they would answer. Do they have to love their neighbour? yes to a point (we know that we have passed from death to life because we love one another -1 Jn 3:14). So the Protestant really believes that we are justified by the attitude of our heart, by faith in God and by repentance, which is exactly what the Catholic believes.
The difference is imputed vs. infused righteousness. St. Paul argues in the beginning of Romans that by having faith we are justified by the 'free gift of God' and that 'Christ died for us' (5:8) HOWEVER in my NRSV commentary the writer says 'the verse should NOT be interpretted 'Christ died in our place' (protestant doctrine of Penal Substitution), but it is better interpretted 'for our benefit' (Catholic doctrine of Christus Victor). How could Christ's vicarious death on the cross mean the final salvation of some when St. Paul writes "just as one man's trespass led to condemnation for all, so one man's act of righteousness leads to justification and life for all" (5:18) - Now unless you're a Quaker and believe in Universal salvation, then you have to accept the Catholic view that Christ died for all, and did so 'the many WILL be made righteous' (5:19). This is showing justification as a process, it is ongoing, and it is Christ's faithfullness which will be infused into us. St. Paul's whole point is that Christ died to make it possible for us to be like him, as St. Irenaeus says 'he became what we are that we might be what he is'. The next theme of Romans is life in the Spirit. St. Paul says "just as Christ was raised from the dead... so we might walk in the newness of life" (6:4). He continually warns us 'do not let sin exercise sin in your mortal bodies' (6:12), and why would he if we are justified by imputed righteousness. Next he writes "become obedient from the heart to the form of teaching to which you were entrusted" (6:17). He sums it up at the end of his argument in chapter 6 with "you have been freed from sin and enslaved to God, the advantage you get is sanctification. The end is eternal life." - The end of sanctification - being made holy - is 'eternal life'. The next verse of course we all had to memorize for sunday school, but the verse before is really the key to the argument.
For Paul the cross means the Possibility of Salvation, it is salvation from 'the law' he writes "you have died to the law through the body of Christ...in order that we may bear fruit for God" (7:4). The big point is 'we are slaves not under the old written code (ie. the law) but in the new life of the Spirit" (7:6). The other big conclusion, the message of Romans as I learned today was that Christ died "so that the just requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit" (8:4)
This is the message of Jesus as well, that we are to love God, and love our neighbour. Finally I have a view of Romans and St. Paul that fit with the rest of scripture. The role of the Holy Spirit is justifying us/sanctifying us. The division of sanctification and justification did not exist till John Calvin in the 16th century, it is foreign to scripture as well. If God just wanted us to believe, why did Christ tell the rich young ruler 'you know the law' when he asked how he was to be saved. The purpose is that through Christ and the work of the Holy Spirit, we can be made righteous. That makes sense of all the verses that say no impure people will enter heaven etc, and also proves purgatory. This has been quite a breakthrough in the Catholic - Protestant search, and it makes alot of sense. So we are justified by a 'heart-faith' alone (Peter Kreeft's term) but we are sanctified by the Holy Spirit who leads us in love and good works, and we are then judged on our love. as Matthew 25 says, did we care for the poor, the lonely, the needy.