"But when the set time had fully come, God sent his Son, born of a woman, born under the law, to redeem those under the law, that we might receive adoption to sonship." (Galatians 4:4-5)
Paul develops the contrast between the heir under age, who is no different from a slave though owner of everything, and the heir who has come of age and received full standing. So we were, enslaved under the elemental spiritual forces of the world. But when the set time had fully come, God sent his Son, born of a woman, born under the law, to redeem those under the law, that we might receive adoption to sonship. The fullness of time: history was moving toward this moment, the whole of the Old Testament a preparation for the sending of the Son. Born of a woman: the Incarnation is stated in the most human terms, reminding the Galatians that the Son entered fully into the condition of those he came to redeem. The Catechism draws from this verse its teaching on the Incarnation as the act by which the eternal Son became subject to the Law he himself had given, in order to free those imprisoned by that same Law (CCC 422).
Because they are sons and daughters, God has sent the Spirit of his Son into their hearts, the Spirit who calls out Abba, Father. So they are no longer slaves but children, and since children, then heirs. Paul is astonished that they want to turn back to the weak and miserable elemental forces to which they had been enslaved. He pleads with them personally: become like me, for I became like you. He reminds them of his first visit, when despite his illness they welcomed him as if he were an angel of God, as if he were Christ Jesus himself. Where has that goodwill gone?
Paul uses the allegory of Hagar and Sarah: Hagar the slave woman represents the covenant from Sinai that produces children for slavery; Sarah the free woman represents the Jerusalem above, our mother, which produces children who are free. The child of the slave was born according to the flesh; the child of the free woman through the promise. We are children not of the slave woman but of the free woman.
Brothers and sisters, God has sent the Spirit of his Son into your heart, the Spirit who cries Abba, Father. You are not slaves approaching a distant deity with a list of requirements. You are children calling to a Father in the intimacy of the Spirit. When you pray today, begin there. Abba. Father. The Spirit in you is already praying it.
Abba, Father. You sent your Son in the fullness of time to redeem us, and you sent your Spirit into our hearts so that we might cry out to you as children. We are no longer slaves. We are heirs. Let us live from that freedom and that inheritance. Through Christ our Lord. Amen.