as to zeal, a persecutor of the church; as to righteousness under the law, blameless. (Philippians 3:6, ESV)Paul continues in mounting his evidence that if confidence in the flesh were to be had... he would out do any of the others that might be preaching the necessity of these things. He does this by now saying that he wasn't just a dull, humdrum follower of the Law. He was all-in! He was full of that righteous energy to do what the Law prescribes. This zeal was so extensive that he worked to fight against those who stood against it. As to the Law itself, there wasn't one point he had missed: he was blameless.
In other words, if there was a righteousness to be earned under the Law, he could have done it.
This means that when he teaches these Christians to abandon the righteousness that can be earned under the law, it isn't because of his own self-preservation. It isn't because he is lacking in an area. It isn't because he wouldn't measure up to the standard.
He wants to make sure they understand that he has been on the other side of this argument.