Many non-Christians attempt to counter claims that people are sinners by pointing to the Old Testament. If homosexuality is a sin, they say, then also wearing mixed fibers is as well (Lev 19:19). My response has always been that there is a difference between symbolic law (law meant to distinguish God’s chosen nation) and moral law. They never believe that this is a Biblical concept. So here is atheist Bart Ehrman explaining the difference (perhaps they will believe him). From Peter, Paul and Mary Magdalene:

In my view, the easiest way to solve the problem [of contrary statements about “the law”] is to say that Paul somehow imagines that there are two basic kinds of laws given in the Jewish Scriptures. There are some laws that are meant for Jews to show that they are members of God’s covenantal community, including the laws mentioned above, of circumcision, kosher diet, Sabbath observance, and so on. These are laws that make Jews Jewish. But salvation in Christ, for Paul, is not for Jews only; it is for Jews and Gentiles. Gentiles are not expected to become Jews in order to be right with God. If they had to do anything of the sort, it would show that the death of Jesus itself was not sufficient for a right standing before God. But it is the death of Jesus alone that makes a person right with God. Gentiles who think they have to become Jews (for example, by being circumcised) have completely misunderstood the gospel.

There is, however, the other kind of law found in Scripture. This is the kind of law that applies to all people—for example, not to murder, not to commit adultery, not to bear false witness, and to love your neighbor as yourself. Everyone, Jew and Gentile, needs to keep these laws. Those who are in Christ are able to keep these laws because the Spirit of God empowers them to do so. So it is not by keeping the law that one is right with God. But one who is right with God will keep the law, at least the law that is designed for all people, though not the law designed to show who is Jewish and who is Gentile.

There we have it. Paul talks about two kinds of laws. The task Christians face is to correctly distinguish between the two.