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. In the same way we also, when we were children, were enslaved to the elementary principles of the world. But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, “Abba! Father!” So you are no longer a slave, but a son, and if a son, then an heir through God.
That first phrase is interesting, saying that a child is no different from a slave! Yet in some ways, especially in this culture, it is true. The child might have better living quarters, and better treatment, yet the child does not have any of the inheritance. They are bankrupt, just like the slave. Yet in another sense, they have the promise of the ownership of a great inheritance. Even though they possess the inheritance, they cannot use it as they wish, but are under the authority of guardians until they reach a time that the inheritance is theirs to do with as they please. They are under authority, just like a slave is.
So rejoice. Rejoice that though we are under authority, we know that as sons, the inheritance is ours. That is the promise. And as children of God, we love him, we cry out to him, Daddy! Father! We love you!
So while we are both enslaved in some ways, our enslavement is with a promise of an inheritance. A promise of a wonderful future. A promise, in this case, not to have that fulfillment when our father is gone, but to have it when we meet Him face to face. A promise that should lead us to joy in all circumstance.