In this series of sermons, “Co-heirs with Christ,” we have so far learned what it means to be an heir of God by looking at the examples of Abraham and Jacob. It is, especially, interesting to see the differences between Abraham and Lot, and between Esau and Jacob—one being an heir and the other not being an heir. Both Abraham and Jacob were blessed in spite of their weaknesses and mistakes, and God trained them according to the purposes he had for them. And both lived as blessings to others: learn well what it means to be a blessing.

Now, we will go on to the case of Ishmael and Isaac. This is a little complicated because another vital biblical issue—namely, how the Law should be understood under the new covenant—is involved in its discussions. First of all, read the following words found in Galatians:

For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. His son by the slave woman was born in the ordinary way (literally, according to flesh); but his son by the free woman was born as the result of a promise (literally, by promise) (Gal 4:22-23).

These two sons are compared according to their birth: one was born from “the slave woman,” and the other from “the free woman.” And the son of the slave woman was born “according to the flesh,” and the son of the free woman “by promise.” If you are familiar with the stories of these two sons in Genesis, it is easy to know what this means. Sarai said, one day, to her husband:

The Lord has kept me from having children. Go, sleep with my maidservant; perhaps I can build a family through her (Gen 16:2).

Note how she spoke about the promise: “The Lord has kept me from having children.” She was not sure whether God was really willing or even able to give her a child, so she spoke against the promise. Abram agreed and did as she said. Compare this with what God said:

I will make your offspring like the dust of the earth, so that if anyone could count the dust, then your offspring could be counted (13:6).

Note that in the promise, it was God who said he would do it, but in the above act of flesh, it was Sarai and Abram who did it. This is a big difference as Paul points out in the above passage. The one who was born according to the flesh was not to be an heir, but the one by promise was.

Now you, brothers, like Isaac, are children of promise. At that time the son born in the ordinary way (literally, according to the flesh) persecuted the son born by the power of the Spirit (literally, according to the spirit). It is the same now (Gal 4:28-29).

Here, Paul throws in the expression “by the power of the Spirit (NIV)” which in Greek literally means “according to the flesh” (not necessarily referring to the Spirit of God) as opposed to “according to the flesh.” Obviously, it prepares for the discussions of freedom and the two lifestyles in the following chapter.

But what does the Scripture say? "Get rid of the slave woman and her son, for the slave woman's son will never share in the inheritance with the free woman's son " Therefore, brothers, we are not children of the slave woman, but of the free woman (vv. 30-31).

We believers were all, like Isaac, born according to the spirit, and are children of promise. And those who were born according to spirit live also according to the spirit. Children of promise live also by promise. Such are the heirs of God who enjoy the privileges and rights promised to them.