Charles Spurgeon Commentary Galatians 4:25

"Now this Hagar is mount Sinai in Arabia and answereth to the Jerusalem that now is: for she is in bondage with her children." — Galatians 4:25 (ASV)

For this Hagar is Mount Sinai in Arabia, and answers to Jerusalem which is now, and is in bondage with her children.

It is old Judaism coming from Sinai, This do, and you shall live, and all the children that are born under it are children of nature, and they are not the children of promise.