‘And because you are sons, God has sent forth the Spirit of his Son into our hearts, crying “Abba, Father”, so that you are no longer a bondservant but a son, and if a son then an heir through God.”
Having adopted us as sons God has ‘sent forth the Spirit of His Son' into our hearts. We receive the Spirit of sonship. Previously He sent forth His Son to redeem us, now He sends forth His Spirit to institute and guarantee our sonship. And we note that it was God Who sent them forth causing us to look to the Father, and say ‘abba, Father'. Thus the whole of the Trinity is involved in our salvation.
The result of the Spirit's work, as we are born of God, and sealed by the Holy Spirit, is that our hearts are filled with certainty (although there may be occasional doubts with some) and we gladly and wholeheartedly cry “Abba, Father”. ‘Abba' is the Aramaic word meaning father which is used fondly by a son to address his father. It is a close and intimate term, and Paul perhaps felt that it was the only term in his experience which could quite express what he wanted to say. But while being intimate, in those days it also had to be said with deference by a full grown son, even to an earthly father.
(It must be an occasion of gratitude to us that we can so address Him, but never an occasion for over-familiarity. For He is the high and lofty One, Who inhabits eternity, Whose Name is holy).
Or perhaps Paul had in mind, and knew that his readers knew it too, that this was how Jesus Christ Himself had addressed His Father when in the deepest distress (Mark 14:36). Thus he may be saying, ‘When the Spirit of His Son enters your heart He will pray through you with the same intimacy as Jesus had.'
And because we have received the Spirit of His Son, through Whom Christ indwells our hearts (Galatians 2:20), our lives will reveal the fruit of our sonship (Galatians 5:22), for we will not respond to the flesh, which was under the Law, but to our new life in the Spirit. We will see following the flesh as for babies. As true sons of God we will want only to follow the Spirit.
Paul spoke similarly to the Romans when he said, ‘For you did not receive the spirit of bondage, again to fear, but you received the Spirit of adoption whereby we cry “Abba, Father” (Romans 8:15). Here too there is freedom from the bondage of the Law, a bondage which produced fear because the Law was broken and could only condemn. And there is also the freedom of adoption as sons of the Father, all fear having been removed because we have been redeemed from the curse of the Law. And he adds, ‘The Spirit Himself bears witness with our spirit that we are children of God” (Romans 8:16). So those who are His have the witness within, the witness of the Spirit, and thus are aware that they are ‘heirs of God, and joint-heirs with Christ', ready to endure with Him, inheriting first the suffering that comes from being those who obey God, and finally the glorification that follows (Romans 8:17).
‘You are each no longer a bondservant but a son.' The appeal is now in the singular, speaking to each individual. Galatians 4:1 had said that the heir did not differ from a bondservant. He was treated like a bondservant even if he was the heir. Thus now Paul declares we are no longer in that state because we are sons. We have grown up and been declared adult. Therefore we should behave as sons. We should no longer allow ourselves to be in bondage to anything which is claimed to be ‘necessary for salvation'. Rather should we allow Christ to live out His life through us, following His example in all things.
‘And if a son then an heir through God.' And if we are in Him we are not only a son but an heir, an heir of the promises made to Abraham, an heir of all God's promises for the future, a co-heir with Jesus Christ of the glory that is to come. And as always with an heir the benefit is not earned but is a gift of free grace.
‘Through God.' So then he cautions them to remember that this is not because of their self-worth or deserving, it is ‘through God'. They owe it all to Him and are therefore totally indebted to Him.