Ascending steps in the dealings of God. When a person accepts Jesus, the change doesn't happen all at once. Because God's people were not ready for the final step, the law was given to prepare them for the coming of Christ. Paul tells them and us that faith in Jesus takes up where the covenant with Abraham left off. Abraham had faith that this covenant would take place.
Faith then becomes a bridge between the two.
Paul then brings in baptism to complete the picture. We have infant baptism to bring our children into the faith of the church and confirmation to have the young people confirm what their parents had promised for them earlier. With this baptism we become one in Christ. The infant becomes a recipient of God's grace the same as the person who is 90 years old. If a person is a foreigner, has different colored skin, a different national origin than the majority of those around them, if they are male or female, slave or free, it doesn't matter.