God knows that human beings cannot easily understand what we cannot see.  So, throughout the Bible, God uses examples from everyday life to help His people to grasp eternal truths.  For example: the animal sacrifices of the Old Testament help us to appreciate the sacrifice of Christ; the tabernacle and temple of the Old Testament help us to grasp how sinful people can worship the holy God.  In the same way, Jesus used parables to explain how to enter His kingdom and grow as a disciple.
 
Likewise, Paul takes the example of a written covenant – the Greek word used referred to a ‘Last Will’.  This is a legal declaration of how a person wishes his or her possessions to be disposed of after death.  It is a covenant which is binding.  So Paul uses the example of human covenants to explain that God’s covenant promises cannot be altered by people or lose their meaning over time (Galatians 3:16-18).  

What God has written stands until the action is completed.  And the Old Testament laws all find their fulfilment in Christ (2 Corinthians 1:20).  Paul’s Jewish readers needed to know that the ‘law’ they respected so much was not abandoned by Christ but fulfilled by Him because Christ is the fulfilment of the law (Romans 10:4).
 
We sometimes forget that the Bible is more than nice thoughts, good ideas and helpful advice.  No, the Word of God is permanent and cannot be bent to our whim: all His instructions must be obeyed and whatever He promises to give will be delivered.  In the same way that we are used to living in a cause-and-effect world, God expects us to understand that our actions have consequences.  But also we should expect undeserved love and forgiveness from the Lord who will fulfil His promise to forgive those who repent.