It is not clear exactly what Paul meant with this phrase. Some argue that the law served to restrain sin or to deal with the results of sin through its various sacrifices. More likely and in keeping with what Paul says elsewhere in his letters, the law came to define sin by revealing God’s character and demands (Romans 3:20; Romans 4:15; Romans 7:7, Romans 7:13).1 Having defined sin, it also serves to increase sin (Romans 5:20).2 When human beings know what God wants and still choose to disobey him their culpability and sin before God is increased.
19 Why then the law? It was added because of transgressions, until the offspring should come to whom the promise had been made, and it was put in place through angels by an intermediary.