Why is it foolish to say you are doing your best by keeping the commandments?
It is foolish because the Law of Moses, the letter, can only condemn and pronounce guilty; it has no life-giving power. Commandment-keeping cannot produce spiritual life. Only grace, the Spirit, gives life. So trusting in commandments for 'doing your best' misses God's provision of life through the Spirit.
Les explains that when Paul contrasts law and grace, the letter refers to the Law of Moses. The law could do nothing but condemn; it had no life-giving power whatsoever. That is why it is foolish to say, 'I'm trying to do the best I can, and I'm keeping the commandments.' The commandments can only kill and pronounce a person guilty. Grace, however, is epitomized by the Word, the Spirit, and the work of the Holy Spirit, which gives life. Les also points to the eternal state, where we will no longer be flesh and blood but flesh and bone, with life now in the Spirit. The same Spirit is the power from on high, the Holy Spirit who gives life.
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.