The gospel of grace is founded upon the gift of God's righteousness and justification because of what Jesus did to bring so great a salvation. It is not based on a merit system that brings blessings and approval for your good works while withholding the same because of poor performance. And while works can be good and profitable, it certainly isn't the mechanism by which we are made heirs, nor does it secure salvation or justification. This occurred by grace alone, according to a promise from God ... the hope of eternal life.
Paul had been tied to a religious system of works which highlighted his performance. He counted it all as rubbish so that he "may be found in Him, not having a righteousness of my own derived from the Law, but that which is through faith in Christ, the righteousness which comes from God on the basis of faith."
Religious zealots will resist ... but it is all about the reality of "Christ in you, the hope of glory." The "proof" of such a faith which results in being justified, saved, and made righteous is found in an empty tomb and a risen Savior. While there are eyewitnesses to this event who have recorded it in writing, we have an even greater witness who is unseen and often ignored. It is the Spirit of God who inwardly bears witness to us of the truth of a risen Jesus and that we are indeed children of God.
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