Jewish-background Christians found it difficult to believe that the law given to Moses was not God’s ultimate authority.  They could not see how Jesus had greater authority.  Paul explained that God’s promise to Abraham was an unconditional covenant which would be fulfilled by Jesus, whereas the law came almost 500 years later and was a conditional covenant.  The condition for the law was that God would bless His people if they were obedient but would curse them for disobedience.  You can read the details in Deuteronomy 28:1-68.
 

Religion loves rules.  Its adherents feel that they are earning God’s favour, and officials have power over the people by demanding that they keep the laws or face punishment.  But faith in Jesus Christ is different.  He is the focus of God’s promises which came before the law was given to Moses (2 Corinthians 1:20).  

What makes a person a child of God?   Paul had grown up to believe that circumcision and keeping the Old Testament law had secured his place in God’s family (Philippians 3:4-6).  But after Paul met Jesus, he realised that Jesus Christ had fulfilled and replaced the law.  Only faith in Jesus could bring people into His family (Galatians 2:15-16).  That meant that Gentiles (non-Jews) could be welcomed by God as equals with Jewish-background believers (Galatians 3:28).  But anyone, including Jews, who refused to believe that Jesus died as a sacrifice for their sins, could not be included in Go

The young heirs of a wealthy estate in the Roman Empire had no power or authority; they could not manage the estate or benefit from it directly.  Even though they would inherit it all one day, as children they were under the authority of their father’s servants.  They had as much say in what happened as the lowest slave.  In the same way, Paul says God’s ancient people were under the authority of the law before Christ came. They had no freedom to be true heirs of the Kingdom until they received Christ and grew up spiritually (Galatians 3:23-25).