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.
The coming of Christ into the world (and, of course, the death He died must be held to be implicit in that coming, as its significant content) is said here to have had a twofold effect: (a) to redeem them that were under the law, and (b) that we might receive the adoption of sons. Not merely is there the rescue from bondage but also there is the making of slaves into sons. Nor does this happen consecutively, but simultaneously. The rescue from slavery is of a sort that it inevitably makes us into sons. This is the new position, the new status, that has been given us. And for this, and because of this, He gives His Spirit (6). The words 'sent forth' in 6 are exactly the same as 'sent forth' in 4 - so that, in the fullness of the time God sent forth both His Son and His Spirit. It is true, of course, that historically, there was a time lapse between these two 'sendings', between Bethlehem and Pentecost; but we know also that theologically they both belong essentially together, and the one is precisely irrelevant without the other. The sending forth of the Spirit into men's hearts is the logical and ultimate fulfilment and completion of the movement of eternity which constitutes the gospel. We could put it like this: the fact of the Incarnation taking place once-for-all in history has necessary and inevitable implications in the here and now. Christ has entered into human life, once for all; but this necessarily means that He enters into our lives now by His Spirit hence the parallel statements in 4 and 6. As John Stott puts it, He sent His Son to secure our sonship in the family of God; His Spirit, to assure us of our sonship, the one, to give us the status of sons; the other to give the experience of it.