Paul uses the picture of Abraham’s two sons Ishmael and Isaac to contrast those who advocate living according to the law to those who live in the freedom of faith (v21-23).

The picture here is of a son in a poor position as a child of a slave, which speaks of those under the law and a son in a much greater position as a child of the promise, which speaks of those who have come to Christ in faith in God’s promises (v24-28). Going back to a focus on the ceremonial law may appear more religious and spiritual, but Paul saw it as a step back to slavery.

The problem Paul sees here is like how Ishmael the son of a slave persecuted Isaac the son of the promise, so too do those going back to the ceremonial law persecute those who seek to live in freedom as the children of the promise (v29-31). This continues to happen today.

In Christianity there are rules, the Ten Commandments still apply, but our primary focus has to be on our relationship with Christ which is through grace. Sadly there are those who prefer to focus on rules, who lose sight of grace and lose the joy of their relationship with Christ. Sadly they can become like Pharisees, always pointing out the faults of others and adding additional rules that must be followed. Beware of such people, there are plenty about.