What could the law actually do for a person?
The law could do nothing but condemn a person and pronounce them guilty. It had no life-giving power whatsoever, so keeping commandments could not produce life. Scripture contrasts the letter, which kills, with the Spirit, who gives life; the law therefore exposes guilt rather than supplying spiritual life.
Les explains that Paul's New Testament ministry was not of the letter—the Law of Moses—but of the Spirit, for the letter kills while the Spirit gives life. The law could do nothing but condemn; it had no life-giving power whatsoever and could only pronounce a person guilty. Les says people are foolish to claim they are doing their best and keeping the commandments, because the commandments can do nothing but kill. Grace, epitomized by the Word and the Holy Spirit's work, gives life; that Spirit is the power from on high.
Who also hath made us able ministers of the new testament; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life.