There are many ethical implications in the writings of St. Paul, which have been the subject of literally thousands of volumes of commentaries. Among the most important notions for Christian ethics is that of salvation by faith:
Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith. Therefore we conclude that a man is justified by faith without the deeds of the law. (Rom 3.27-28.)
This notion is important because it emphases inner motivation rather than external act. If, for example, one obeys the laws against assault and murder for fear of not getting caught, that is not an ethical, but a self-serving act, even though it is not outwardly distinguishable from the behavior of someone who avoids violence from moral conviction. St. Paul argues that because outward acts are no guarantor of ethics, rather than judge others, our main ethical job is to investigate our own motivations.