Philippians 3:6 (KJV) Concerning zeal, persecuting the church; touching the righteousness which is in the law, blameless.
Paul stated that he was blameless when it came to keeping the law. According to the law, he was righteous. But then how could he say there was no good thing in him?
Romans 7:18 (KJV) For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.
And why would he call himself wretched?
Romans 7:24 (KJV) O wretched man that I am! who shall deliver me from the body of this death?
He realized that even he fell short of God’s glory.
Isaiah 64:6 (KJV) But we are all as an unclean thing, and all our righteousnesses are as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away.
No matter how perfectly we keep the law, no matter how righteous we may be on our own, our righteousness is still as filthy rags before God. We must not trust in our righteousness but we must trust in the righteousness of Jesus Christ. He is our only hope.
Philippians 3:9 (KJV) And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith: