Unfortunately, as humans always tend to do, many in Israel forgot or ignored God's glorious purpose for Law and began to use it improperly -- as a means of proving they were better than others, whether those outside Israel or even the "sinners" among the Jews. To do this, they had to reinterpret the demands of the Law in a way that eliminated the heart requirements and which reduced the external requirements to a manageable and attainable selective list. Then they argued that they were "righteous" because they had kept the law, and considered themselves morally superior to others who had not. We may call this a "Rationalized Righteousness." It is what Jesus opposes in Matthew 5-6, and what Paul fights in Galatians and in Romans
When Paul viewed his pre-Christian self -- the Saul of Tarsus who blasphemed Jesus, assisting in Stephen's execution and "made havoc" of the church -- as "blameless" (Phil. 3:6), I think he was speaking in terms of Rationalized Righteousness. But coming to know Christ destroyed and reversed that mindset. Everything in which Paul formerly trusted was suddenly eclipsed by this Savior who alone truly pleased God in every thought, word and deed. All the religious achievements which previously made him proud became dim, dull and even "dung" as he viewed and contemplated this Savior who lived his earthly life perfectly attuned to the Creator, always acting in loving, trusting, obedient, harmonious relationship with the heavenly Father (Phil. 3:8-11). In the bright light of Jesus Christ, the deficiency of Rationalized Righteousness becomes painfully apparent.
When we came to know Christ, our Rationalized Righteousness was blown to dust. We realized that we will never be righteous based on our own performance, for we continue to come short of God's glory (Rom. 3:23). We saw that we cannot "establish our own righteousness" through our record of obedience and good deeds, and that we needed to submit ourselves to "the righteousness of God" (Rom. 10:1-3). We realized that Christ is the end of law-keeping as a means of attaining God's approval (Rom. 10:4). We saw that we had created a doctrinal system which was manageable and attainable in churchly details, but which ignored many of God's clear commands to us as individuals. We realized that we had deceived ourselves into thinking that we were "righteous" because of our right knowledge and right practice, so that we could actually look down on all our "religious neighbors" (which was how we referred to Christians who did not belong to one of our churches).
We formerly preached ourselves -- "the true church" with all its supposed "marks" and "distinctives." When God shined the light into our hearts, we saw the glory of God in the face of Christ Jesus -- and we knew that we must now preach Jesus Christ as the only Savior of sinners (2 Cor. 4:5-6). We were enslaved to our own misconception that God's approval depended on our correct understanding and performance. When we truly saw Jesus, God lifted the veil from our faces and set us free -- free to truly become all that God wanted us to be (2 Cor. 3:16-18). Yes, like Paul of old, we formerly thought we were "blameless." But Jesus showed us what we really are apart from him. Now we trust in Christ alone, and we rejoice in God's righteousness which he freely gives to all who put their trust in his Son (Phil. 3:8-9).