- sin entered the world through Adam, Romans 5:12
- sin was in the world before the Law, Romans 5:13
- ergo Adam was before the Law
- the Law entered the world 430 years after the promise was made to Abraham, Galatians 3:17 ((to wit, at Sinai, just like Exodus records))
- ergo regarding Romans 5:13 -
- Paul is not saying Cain was given and/or was under the decalogue
- Paul is not saying God is unjustified to judge anyone before Moses
- Paul is explicitly excluding both such claims
i know it's a big block of text and I had to whittle it down to fit in the character limit for one post, but I encourage all of us discussing this to read Chrysostom's view on this. it represents some of the the best scholarship of the early church, and he neatly avoids any of these rabbit trails into presuming the Law came before Moses, which is rife with contradiction to the text of the Bible.
briefly, he sees Romans 5:13 as specifically a statement rebutting early, peculiarly Jewish arguments against the Christian faith and the Christian's dogmatic assertion that they were not under the Law.
the whole homily is easily found online by searching his name + homily 10 Romans, if you are interested.