"Now I say, as long as the heir is a child, he does not differ at all from a slave although he is owner of everything, but he is under guardians and managers until the date set by the father. So also we, while we were children, were held in bondage under the elemental things of the world. But when the fullness of the time came, God sent forth His Son... that we might receive the adoption as sons. Because you are sons, God has sent forth the Spirit of His Son into our hearts, crying, 'Abba! Father!' Therefore you are no longer a slave, but a son; and if a son, then an heir through God." (Galatians 4:1-7)In referencing our progression from childhood, to adulthood, to being the inheritor of all things, Paul is not referring to our natural growth but the growth of the human race. It would be wrong to interpret the words of Paul to mean that we must all start under the law and then, only when mature, enter into the grace that is in Christ Jesus. In our present age we are all born into the age of grace and grace is available to all of us regardless of our stage of development or maturity. Rather, Paul is referring to the growth of the divine revelation among mankind.
The promise given to Abraham was given almost two thousand years before the birth of Christ. For tow thousand years mankind lived as minors in reference to the promise of God. The promise was ours but was kept on account for us against a set date when the Father would make it available to us. Though we were to inherit the promise, at the time we differed not from other humans who were in bondage to the "elemental things of the world." This phrase refers to those things that have been set in "order" like an "orderly" march of an army. Such things make up a "common morality" that most nations and people ascribe to and which regulate our lives and our relationships. Such morality that would condemn theft, murder, lying, and many forms of violence has common agreement in many societies and cultures as it is easily seen and understood by the "orderly" nature of the world around us.
The law expresses such a morality and, for nearly two thousand years, we were held in bondage to its commands. The Law was to us "guardians" and "managers" watching over us and regulating our behavior until the day that we might be given what was promised. However, now having received the promise we no longer need those guardians and managers. Through God's promise we have been give the grace and strength to live up to a higher standard of morality and no longer need the law and its bondage to lesser things. We are now full-fledged daughters and sons of the Most High. All that He has, and all that He has promised in the past, is now ours in abundance. We are now living in the fullness of time.
However, many still live in bondage; as slaves to an old way of living. They have yet to experience the adoption as sons and daughters; sons and daughters of God. They have yet to receive His Spirit into their hearts; the Spirit that bears witness with our spirit and empowers us to live godly lives. The transition from bondage to sonship all begins with our relationship with Jesus. If we will repent and believe the Gospel, believing in Jesus, then we too will make the transition to sons and daughters and will experience with the rest of God's family what it means to be the inheritors of all things.