I mean that the heir, as long as he is a child, is no different from a slave, though he is the owner of everything, but he is under guardians and managers until the date set by his father.Galatians 4:1-2
Paul illustrates his point made in chapter 3 that, in the progress of revelation, the Law was given to Israel, but it was not the final revelation. It was a master that held captive those to whom it was given until faith would be revealed (Galatians 3:23-24.) The coming of Jesus and justification by faith was what the Law was pointing to, but from the perspective of the Mosaic covenant, was still in the future.
The Law then was to point to the coming faith and revealed the true need of humanity just as the guardian in pre-modern times took care of and instructed the children of the master. Several aspects of the law reveal the deepest need of the human condition: sin, the rebellion against the rule of God over all aspects of human life, brings consequences appropriate of the disobedience. In Romans, Paul summarizes that “the wages of sin is death.” (Romans 3:23.) The law, in all of its goodness was never meant to be the purveyor of salvation, but to point to both the need and the coming salvation. Even the sacrifical system spoke to the coming need of a more permanent sacrifice since the daily and annual sacrifices never came to an end because they were satisfactory forever. Rather, the sacrifices continues because they could not cleanse the concencience of sin or remove the guilt; sacrifice can only remind the bearer of their failures.
However, where the law was unable to bring salvation, it was able to point to where salvation would come. That is why Paul uses the ancient cultural practice of an heir and their guardianship. There would be a day where the father gives to the son authority over the household, but until then, was in bondage to the guardian even though he is the son. The coming date in which the law would end its relationship as a guardian is when Christ came and faith in him being the end to which the law was pointing.
Looking back at Christ’s redemption by his death, we first see our need of a savior when we look at God’s law and see that we are deficient in keeping it. We do not love God as we ought and, left to ourselves, will seek to please ourselves instead of Him. Once we recognize our rebellion, we can now turn to Christ and see that our rebellion and the infinite offence we made against God can now be settled in Christ by faith in his cross-work alone.