1 “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, 2 but he is under guardians and managers until the date set by the father. 3 So we too, when we were children, were held in bondage under the elementary principles of the world. 4 But when the fullness of the time came, God sent His Son, born of a woman, born under the Law, 5 so that He might redeem those who were under the Law, that we might receive the adoption as sons and daughters.6 Because you are sons, God has sent the Spirit of His Son into our hearts, crying out, ‘Abba! Father!’ 7 Therefore you are no longer a slave, but a son; and if a son, then an heir through God.”
Elementary principles of the world: Paul uses an analogy of slaves and children to express what life was like under the law. Before Christ came, people were under the law and consequently enslaved to sin (see Romans 7). We lived according to the flesh and according to principles of the world. But Christians are different now. We are now children of God and not in bondage to sin.
But when the fullness of the time came…born under the Law: Jesus was born under the law. This means that He lived and taught under the law. Consequently, the New Testament doesn’t start in Matthew 1, but at Christ’s death (Luke 22:20; Hebrews 9:16). This is why Jesus taught the impossible standards of the law perfectly during His ministry (Matthew 5, 6, 7). He wanted to shock His hearers so that they saw their need for Him. He could redeem those under the law if they knew their need for His grace.
Because you are sons: God has given the Holy Spirit to His children. Just as the Spirit calls out to the Father, so can we call God Abba. We are saved to have an intimate relationship with God.