1. We were all slaves once (1-3)
An underage heir of a large estate is no different from a slave of that estate. Even though an owner's child will someday be the owner, one is under the constant watchful eye of one's parents and told what to do, how to dress, when to eat, and when to sleep. The same applies spiritually. Once we were under the spiritual forces of the world. We were controlled by our sinful nature. Just as a child can't escape the control of their parents, we couldn't escape being controlled by our sinful nature.
2. Not a slave nor a child, but an heir! (4-7)
At just the right time, God sent His son in order to redeem us, who were under the law. What was the result of this change? We went from being like a young child or a slave to being adopted as God's children with the full rights of an heir. God also sent His Spirit into our hearts, and, by it, we cry 'Abba, Father.' It is only by His grace that we are made to be heirs of God's kingdom.
Application: Lord, thank you so much for adopting me as your child. Thank you for making me an heir of your kingdom.
One Word: I am adopted as God's child