Chizuk Emunah (Pt 2) Under the Microscope: Chapter 83

1 Corinthians 5:1, Paul reproaches his followers that "there be fornication among them, and that one of them had committed incest by marrying his father's wife."

If Paul, as all his writings indicate, considered the Christians dispensed from observing the Mosaic Law, where was their boundary of religious duties or transgressions, seeing that Jesus had not promulgated a new code of laws? Surely, no permission had been granted by the founder of Christianity, so that his followers should observe part of the Mosaic laws, and reject the remainder. 

Troki can be forgiven of contradicting rabbinic tradition, since he is a Karaite. In Rabbinic Judaism, Maimonides himself made the distinction between the Mosaic Law and the moral law. There are certain temptations, such as the temptation to eat pork, which a Jew is allowed to entertain, stating that he would love to eat that pork, but his creator forbids him from doing so. A Jew is not allowed to entertain temptations to violate the moral law. He cannot say that he would love to have an affair with a woman, but his creator forbids him from doing so. This shows that Maimonides believed in such a distinction.

Paul did as well. He writes in Romans:

For all who have sinned without the law will also perish without the law, and all who have sinned under the law will be judged by the law. For it is not the hearers of the law who are righteous before God, but the doers of the law who will be justified. For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them on that day when, according to my gospel, God judges the secrets of men by Christ Jesus.
(Romans 2:12-16)

Gentiles, who are not bound by the Mosaic Law, are still bound by a moral law, which is written on the hearts of everyone. Not only does Paul not oppose the moral law, he supports it. Paul repeatedly warns his readers about avoiding sin and encourages them to seek holiness. This would be puzzling if Paul believed that the moral law had been abrogated.

Another take is from the rabbis, who said that non-Jews are to follow the Seven Laws of Noah. The fourth law says not to engage in sexual sins. Even if the Mosaic Law were totally abolished, this moral law remains, since it precedes the Mosaic Law.