Martyn-Lloyd Jones emphasizes that the purpose of the Law is to define and reveal sin, making it clear that humanity is without excuse. The Law serves as a 'school-master' to lead individuals to Christ, highlighting their inability to save themselves and the exceeding sinfulness of sin. By understanding the Law, one recognizes their own sinful nature and helplessness, ultimately pointing to the need for salvation through faith in Jesus. The summary of the Law is to love God and neighbor, a standard that no one can fully meet, reinforcing the necessity of grace for redemption.