What does the casting out of Ishmael teach us to do with the law and legalism?
The casting out of Ishmael teaches that law and legalism in all their hideous forms must be totally separated from and cast out. Just as Ishmael was sent away with no contact with Isaac, believers must not return to the law's weak and beggarly elements or bondage, but remain separated from legalism.
Les explains that Ishmael was cast out into the wilderness so that there was no contact between Isaac and Ishmael. The lesson is that this is exactly what we have to do with the law and with legalism in all its hideous forms: we have to be totally separated from it, and we have to cast it out. He notes that the law was carnal, fleshly, and weak from man's side, though from God's point of view it is spiritual, perfect, and holy. In Galatians 4:9, Paul writes to Gentile believers who were saved by grace but were tempted to go back under the law, asking why they turn again to the weak and beggarly elements and desire again to be in bondage, which is the law. Romans chapter 8, verses 2 and 3, says practically the same thing.