Summary: More times than we would probably like to admit, we slip into legalism. It can happen when God takes too long to answer a prayer, or answers in a way we don't expect. We take over the reigns, and it's a mistake!
To continue Paul’s arguments against legalism he turns again to a story in the Old Testament. This time it is the children that God promised to Abram. The issue is whether you let God fulfill His promise of righteousness in you, or whether you want to take the controls and do it yourself. Paul wants us to think about what that really means if we take it to its fullest extent.
21 – 30
The story begins back in Genesis 15 (quickview) . God had promised to make Abram a great nation back in chapter 12. But by chapter 15 they had had no children. So Abram asks God about it and God promises that “one who comes from your own body will be your heir.” (Genesis 15:4 (quickview) ).
It is at that point where Abram “believed the Lord, and He credited it to him as righteousness.” It is an awesome promise and a wonderful response. Funny thing about those promises of God: we expect them to be fulfilled right away; God seems to be on another timetable.
Ten years later Abram and Sarai are still waiting for a son. Sarai comes up with a brilliant idea—if God isn’t going to come through for us, let’s fulfill His promise ourselves! This happens to us all the time. Our impatience overtakes our faith and we figure that “God helps those who help themselves” and so we just make it happen.
What happened here should be an object lesson for all who want to hurry God. Abram married Hagar, Sarai’s Egyptian slave, and had Ishmael. Ishmael became the father of all the Arab peoples who have been Israel’s bitter enemy ever since. You’ve got to wonder if Abram might have thought—“if I had this to do over again…”
Later, when God came to visit Abram by Sodom, he told him that they’d have a child by that time next year—and they did—Isaac. So in verse 23, Abram’s flesh produced Ishmael, but God’s miracle promise produced Isaac.
Paul explains in verses 24 and following that these births also are a picture, as are most things that happened in the Old Testament. This picture is the Law and the promise of God to bring about our righteousness by faith in God’s promise.
Mt Sinai is where the Law was given, but is represented by Jerusalem—the current seat of the Jewish law. “She is in slavery” (verse 25) Paul says in trying to be good through futile human attempts at righteousness.
Verse 26 promises a New Jerusalem (see Revelation 21 (quickview) )—God’s kingdom on earth. From God’s kingdom comes God’s miracle—our being “born again.” The reference in verse 27 is from Isaiah 54 (quickview) . It comforted the exiles in Babylon that God would restore them to better things. So too, God has restored to us better things than when we were ruled by the flesh and our abilities.
Verses 28 and following tell the story in Genesis 21 (quickview)  of what happened when Ishmael began to persecute Isaac. Sarai had her and him mom thrown out of the camp. The idea is that Isaac (Christian theology of grace by faith) and Ishmael ( Jewish theology of merit by obedience to the Law) are not going on the same path and so can’t co-exist, which is what the Judaizers were trying to do.