The New Covenant gives freedom from the bondage of sin and death because it provides for justification through faith in the sacrifice of Jesus Christ. Sarah, therefore, corresponds to Jerusalem above, and the church is seen as the children, just as miraculously conceived as Isaac was. It is free because of the combination of justification and the Spirit of God, both of which are contained in the New Covenant.
What happened to the bondwoman—the Old Covenant and what it produced? It is thrown out! In a different context, Paul says it is made obsolete (Hebrews 8:13). Why? It has served its purpose. There is nothing complex about it. It was merely a bridge until Christ's coming. The Old Covenant could not deliver, though it served God's purpose for the time it was in force.
But the freewoman—the New Covenant and what it promises—remains and continues to produce.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 27)
Other Forerunner Commentary entries containing Galatians 4:28:
2 Kings 4:8-37