But we know that the law is good, if a man uses it lawfully,

 It was threatening from the Jewish direction. To the Jews there was no book in the world like the Old Testament. Their scholars spent a lifetime studying it and expounding it. In the Old Testament many chapters and many sections are long genealogies; and one of the favourite occupations of the Jewish scholars was to construct an imaginary and edifying biography for every name in the list! A man could go on for ever doing that; and it may be that that was what was partly in Paul's mind. He may be saying, "When you ought to be working at the Christian life, you are working out imaginary biographies and genealogies. You are wasting your time on elegant fripperies, when you should be getting down to life and living." This may be a warning to us never to allow Christian thinking to get lost in speculations which do not matter. 