“What is the kingdom of God? Answer: Simply what we learned in the Creed, namely, that God sent his Son, Christ our Lord, into the world to redeem and deliver us from the power of the devil and to bring us to himself and rule us as a king of righteousness, life, and salvation against sin, death, and an evil conscience. To this end he also gave his Holy Spirit to teach us this through his holy Word and to enlighten and strengthen us in faith by his power.”
-- Large Catechism (Third Part: The Lord’s Prayer), The Book of Concord

Galatians 4:1-7
1 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, 2 but he is under guardians and managers until the date set by his father. 3 In the same way we also, when we were children, were enslaved to the elementary principles of the world. 4 But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, 5 to redeem those who were under the law, so that we might receive adoption as sons. 6 And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" 7 So you are no longer a slave, but a son, and if a son, then an heir through God.