Having said 'sons' in verse 14, he could have concluded directly, 'if sons, then heirs,' in verse 17. But he considered it necessary to strengthen the conviction of sonship. He did this not only by pointing to the mystical birth from God and the reception of the Spirit in the Mysteries of Baptism and Chrismation, but also by the testimony of the innermost feeling, which assures us that we are children of God.
Why are those led by the Spirit of God led by Him? Because they are born of Him, and, being born of Him, they have received Him into themselves. Thus it is evident that those led by the Spirit of God are sons of God. They do not stand in an outward relation to God, but in an inward and living one.
This is accomplished in the Mysteries by the grace of God. Yet the work of life must consist in continuing what was begun in them. How can this be brought to harmony and fulfillment? In this way: those born of the Spirit must also be led by the Spirit. The true sons of God are those who are led by the Spirit of God.
The Apostle has brought the whole matter down to experience, to the testimony of life. This was necessary for him, so that, by conditioning inheritance on sonship, and sonship on the guidance of the Spirit, he might kindle those who desire the eternal inheritance with zeal to walk according to the spirit. For those who walk according to the spirit are those led by the Spirit.
You were born of the Spirit. You received the Spirit in the Mysteries. But this is not enough. You must be led by the Spirit, or guide yourself by the Spirit, and live according to the Spirit. 'Are led' means both that they are led and that they lead themselves. The Spirit of God instills guiding thoughts, feelings, and movements. The believer consents to them, receives them as guidance, and is led by them. Gracious guidance wonderfully joins the power of the Spirit with human free choice.
Chrysostom says, 'By speaking in this way, the Apostle expresses his desire that the Spirit of God should be master over our life, as a helmsman over a ship or a charioteer over a pair of horses. The Apostle imposes such a bridle not only on the body, but also on the soul. He does not give authority to the soul itself. On the contrary, he has subjected its authority to the power of the Spirit. So that those made confident by the gift of Baptism may not thereafter neglect the good ordering of their life, he says: Although you have received Baptism, if afterward you are not led by the Spirit, you will lose the dignity granted to you and the right of adoption. Therefore he did not say, All who have received the Spirit are sons, but, All who are led by the Spirit of God. This means those who conduct themselves throughout their whole life in such a way that they are led by the Spirit. These are sons of God. Since this dignity was also given to the Jews, for it is said, I said, You are gods, and all of you sons of the Most High (cf. Ps 82:6), and again, I have begotten sons and exalted them (Is 1:2), and again, Israel is My firstborn son (Ex 4:22), and Paul himself says, To whom belongs the adoption (Rom 9:4), he therefore explains the difference between that honor and this one. Though the names are the same, he says, the realities are different.'