Who would think that Paul would compare the child of the slave-woman Hagar to the "Mount Sinai" crowd? Mount Sinai was major! Mount Sinai was about power, heritage, and pride! "We are the chosen ones" could be the theme-chant for the people at Sinai!
Paul's words for Sinai have more to do with isolation and slavery than power and pride. It wasn't as if Paul was embarrassed of his heritage. He told the Philippians that if anybody had reason to boast in the flesh, it was him. Paul realized, however, that Sinai was about commands he couldn't keep. When he was reminded of the demands of the law, he came face to face with his failures. When Paul considered Sinai, his reaction was the same as the Hebrews at ground zero – he was fearful! Sinai was awesome, but Sinai lead straight through Jerusalem and on to slavery to the consequences of sin.
"But the Jerusalem that is above is free!" (Galatians 4:26) This Jerusalem began with a woman who was unable to have children until God gave her a promise. She is our mother. She is our heritage. The child born because of the promise is the real child of power, and the inheritance that was promised regarding him leads straight to us and to our justification by faith. That's freedom! That's Life at Work!