Once again, Paul builds upon his message that the Gentiles are just as welcome to the Kingdom of Heaven as the Jews. He talks about heirs (parallels the Jews) as being no different from slaves until they come of age. But because God interceded with his Son, we are all equally adopted into the Kingdom. Not by our actions or by our birthright, but only by the work of the Lord.
Now the challenge becomes putting off the old ways. For the Jews it is to put off the ways of tradition centered around the Law. For the Gentiles it is to be subject to Christ and his teachings. It makes no sense to live like we did as “slaves” when we have become adopted into Christ’s family.
That place in the family comes with many privileges but also responsibilities. To act in according to the way the patriarch would want us to act. We have our free will and our place in the family is secured, regardless of our mistakes and sins. Still, we are expected to work out our sanctification (to become like Christ).