Using the examples of Sarah and Hagar from Genesis and the giving of the law in Exodus, Paul shows that whenever men try to interfere, by their own power, with God's promises, the result is failure. We must be wary of the natural inclination to do any works in order to please or help God.
A continuation in our series through Galatians. The law does not, in any way change the promises of God, but it does shine a light on our sin and show us our need for Christ.
Paul stresses throughout Galatians that salvation is ALL of God and NONE of us. There is nothing that we need to do to help it along, and nothing that we can do to mess it up.