The story of Sarah and Hagar is a picture of two totally different approaches to love and fulfilment, and how to trust God rather than relying on our own ideas and abilities.  God had promised Abraham that he would have a son, but his wife Sarah could not bear children.  Sarah, perhaps seeing her husband’s unhappiness, suggested that he should sleep with her slave, Hagar, and Ishmael was born (Genesis 16:1-4).  Although he grew up in Abraham’s household, Ishmael, would not be the heir (Genesis 17:15-19) because God had promised to give a son to Abraham and Sarah.  When God’s promise was finally fulfilled, it became obvious that He was right after all and Abraham had no reason or right to sleep with the servant girl (Genesis 21:8-12).
 
In these verses, the slave woman, Hagar, represented the law given to Moses on Mount Sinai and was perpetuated in the temple in Jerusalem.  The common thread was that you could fulfil God’s promise by doing the best you can.  But, if achieving righteousness is left to us and our ability to follow the rules completely, we are in slavery.  It is not like that in God’s eternal kingdom - the new Jerusalem – where there is nothing to separate us from Him (Revelation 21:1-5).   

Paul was teaching Christians who were tempted to go back to legalistic religion. ‘Can’t you see, trying hard (and failing) to keep the law only produces slaves?  It is God's grace through Christ which makes people free?’  He begged them not to go back to the slavery of religion which could never earn them the freedom which God gives to His children.  He wanted them to stay firm to the gospel in God’s grace, trusting that Jesus has done all that is needed for people to be accepted by Father God, and then living in the power of the Spirit (Colossians 1:21-23).
 
Whenever we step away from the grace of God through Christ, we move from relationship to rules, from freedom to frustration.  Yet the pressure is strong to do so.  Our fleshly nature wants to satisfy our desire to be in control, but we only succeed in becoming a slave.  Religion offers a route to pleasing God by our own efforts, but the concept is flawed.  We can never please ourselves or God by what we do; we need to receive His grace.  Then having received freely we are to give freely to others in a way which honours the Lord, blesses others and gives us great joy.