The Old Testament has "included" or "enclosed" or "shut up" all of humanity under the umbrella of sin. Not a single person can appear before God on the basis of his own merit or righteousness. The totality of mankind is enslaved by sin and does not have the means to break free from its grasp. By "concluding" that everyone is under the bondage of sin, or under the curse of sin, Scripture shows that something external to mankind has to act to provide a solution that can save man from himself and his sinful nature. This "conclusion" also demonstrates that none of the paths that man has embarked on—primarily justification on the basis of one's own works—are of any lasting worth.
Because all other paths are shown to be futile, the only option for salvation and glorification is the way that Jesus Christ has set forth. There are no other alternatives. Faith in what Christ has done, is doing, and will do is mankind's only hope.
The story of the Israelites is a record of a people whom God chose, set apart, and blessed with incredible blessings and opportunities. But it is also a record of mankind's sinful nature and how illogical it is that a man could stand before God on account of his own innate righteousness. God revealed just a portion of His will and character—the letter of the law—to Israel, and its history powerfully demonstrates that, by himself, man is unable to live up to God's standards.
This should be glaring testimony that some other means is required for man to have a relationship with his Creator. The solution is justification—being brought into alignment with God and His law—on the basis of belief in the Savior and His perfect sacrifice. This marks the beginning of the relationship.
But because faith without works is dead, the way a man lives his life demonstrates who and what he believes in. If he has been justified before God and is being saved, his life will reflect God's mercy, providence, and sovereignty. We are not justified or saved by our works, but if we are justified our works will demonstrate that we are being saved. "Belief" in Christ will be an everyday, continual reality, and true belief will shape every thought, word, and deed.