When we lived in Budapest, passengers who bought bus tickets in advance could board buses without checking in with the drivers.
However, transportation officials randomly boarded buses unannounced to fine passengers without tickets. When this happened, there were always people who moved nervously toward the doors, hoping the next stop would come before the official asked to see a ticket they didn't have.
What a great illustration of Romans 3:20: Therefore no one will be declared righteous in God's sight by the works of the law; rather, through the law we become conscious of our sin.
God's law is like the officials who get on the bus to catch and fine passengers without tickets.
The Law makes us conscious of sin, and we can respond by moving toward the exit doors and hoping we don’t get caught, or we can get a ticket paid in advance .. a ticket Christ purchased for us on the Cross.