In Galations 4, Paul implored the people not to keep returning to the Law, making themselves slaves to the elementary principles of the world that were within the law.
Not only were the people now free from the slavery, Paul said that they were heirs. The heir, Paul said, was no different than a slave when he was a child. Though as an heir, he was the owner of everything, he remained under close watch until he was of age that the father would give him full reign and possession.
According to Paul, mankind was the same way under the Law. They were enslaved to the elementary principals of the world. They had a set of rules to abide by, and were under close watch for punishment if even one of those rules were broken.
But when the time had come for mankind to become heirs, God sent his only Son. Jesus Christ was born of a woman, born under the law. In order to redeem all mankind enslaved to the law, Christ fulfilled the entire law to adopt us as sons and daughters of God.
And now, because we are sons and daughters of God, the Holy Spirit of His Son was sent by God into our hearts to cry unto the Father. Therefore, we are no longer enslaved, but a son! And if we are sons and daughters of God, we then are also an heir through God!
Paul said that in days gone by, when mankind did not know God in His fullness, they were enslaved to those that by nature are not gods. Now that they had came to know God, or rather to be known by God, why would they even want to turn back again to the weak and worthless law? As heirs, why would they want to return back to slaves once more? Paul said that he was afraid that he labored for them in vain, since they kept returning back to the law!
As Christians, we should ask ourselves: Have we remained heirs to the Kingdom of Heaven, or have we brought ourselves back into slavery of the law? Do we now stand watch over others to keep them bound in their chains of slavery by condemning them of the ways they break the law that Paul called "useless?" Or have we gone beyond the elementary principals of the law, conducting ourselves with grace, love, and peace? Do we present ourselves to others as true heirs to the kingdom, or do we hold our hands up before them to show our chains of slavery to the law?