Two sons, the slave and the free. The law and the promise. Read Galatians 4:21-23.
Tell me, you who want to be under law, do you not listen to the law? For it is written that Abraham had two sons, one by the bondwoman and one by the free woman. But the son by the bondwoman was born according to the flesh, and the son by the free woman through the promise.
Galatians 4:21-23 NASB
The Bible often works with 2s. There is a first reference to the law and the flesh. And then there will be a reference to the spirit and the promise. Here we see that the 2 sons of Abraham represented this. The first son born of Hagar, Ishmael, was born under the law and represented the flesh. The second son born from Sarah, Isaac, was the son of the promise.
God gave the law so that we would understand the need for the grace of God given through the promise. Are we trying to live under the law, or are we living in the promise of God through His mercy.