Paul offended nothing against the law of the Jews

ACTS 25:7-8
7 And when he came, the Jews who came down from Jerusalem stood round about, and made many and grievous complaints against Paul, which they could not prove.
8 While he answered for himself, Neither against the law of the Jews, nor against the temple, nor yet against Caesar, have I offended any thing at all.

Note: How could Paul say that he offended nothing against the law of the Jews if he was not a Sabbath keeper? Was he a liar? Or was it rather that he told the truth, and being a Jew, he not only kept the Sabbath, but also the feast days (Acts 18:21; 20:16)?