Why can't the Law give eternal life if it is perfect?
The Law is perfect, and if any human could keep all Ten Commandments, God would grant eternal life. But no person can fulfill it; only Christ Himself kept the Law perfectly. Therefore the Law cannot give eternal life because human inability prevents it, not because the Law is flawed.
Les explains that the Law was not against God's promises; it enhances them. If a law could have given life, righteousness would have come by the Law. The Law was perfect because all the demands of the Holy God were in the Ten Commandments. If a human being could have kept all ten, God would have let him into Heaven, for he would have met God's demands. But the problem is that no person can do it. Nobody can keep those Ten Commandments other than Christ Himself.
Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law.
But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.