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.
Notice the sequence in 5, 6: redemption - adoption of sons - the gift of the Spirit - prayer (Abba, Father). The adoption of sons is something that God bestows - it is the gift of His free, sovereign grace in the gospel - cf 1 John 3:1 - this is God calling those things that are not as though they were (Romans 4:17). This is the inestimable gift of God, that we should be born into His family, and known of Him (9). Now - if this is true, if this is what has happened to us, then, because this is His sovereign enactment and pronouncement, He has sent His Spirit into our hearts. And it is this that enables us to call Him 'Father'. This is the grace by which we are enabled to be what God has made us, in Christ. The challenge implicit in these words is well summed up in the lines of the hymn, Think what Spirit dwells within thee, What a Father's smile is thine, What thy Saviour died to win thee; Child of God, shouldst thou repine? Nor is the challenge needless, for it is possible for freemen to continue to have a slave mentality and a servile spirit. This is what was happening to the Galatians: they were slipping back almost inadvertently, through wrong thinking, into the bondage from which they had been delivered by the gospel. This is the force of Paul's expostulation in the verses which follow.