Why Would You Exchange Being a Son for being a Slave?

The Apostle continues his argument against the Judaizers (Legalists) that were afflicting the Church with their false "Gospel". It is not what we do for God, but what God has done for His people which secures the blessing of God for them. He sent His Son for His people in order to redeem them, and it is He Who has sent the Spirit of His Son into our hearts, out of which comes change of behavior.