Why is keeping the law futile for attaining God's righteousness?
Keeping the law is futile for attaining God's righteousness because human beings cannot keep the law. Even sincere religious people may think they are obeying it, but human effort cannot produce that righteousness. Righteousness is not earned or attained; it must be imputed apart from law-keeping.
Les explains that the religious people in the account were sincere and really thought they were keeping the law, and to the best of their ability they were. But sincerity did not make law-keeping effective for righteousness. When Scripture gets to Romans, it shows how futile this is: human beings cannot keep the law, and therefore cannot attain unto God's righteousness by their own effort. God's righteousness must be imputed. The surrounding context also shows that religious people can be instruments of Satan to hold back the truth of God. Peter and John refused to stop speaking what they had seen and heard, and Jude says believers are to contend for the faith.